This Mushroom Patty Melt is a comforting, diner-style sandwich made completely meatless without sacrificing flavor. Roasted portobello mushrooms are marinated in a garlicky balsamic-soy blend, layered with sweet caramelized onions and melty Swiss cheese, then grilled between crispy slices of rye bread. It’s hearty, savory, and deeply satisfying.
Why You’ll Love This Recipe
It delivers classic patty melt flavor without any meat.
Roasted portobello mushrooms provide a rich, savory bite.
Caramelized onions add natural sweetness and depth.
Swiss cheese melts beautifully for that gooey texture.
Rye bread becomes golden and crisp in the skillet.
It’s budget-friendly and filling.
Easy to customize with different cheeses and breads.
Perfect for lunch, dinner, or casual entertaining.
Ingredients
(Tip: You’ll find the full list of ingredients and measurements in the recipe card below.)
½ cup vegetable oil, divided
1 colossal onion (about 565g or 3 cups when thinly sliced)
4 large portobello mushrooms
2 garlic cloves, minced (about 1 tablespoon)
¼ cup balsamic vinegar
⅛ cup soy sauce
½ teaspoon dried thyme
8 slices rye bread
⅓ cup mayonnaise
8 slices Swiss cheese
¼ teaspoon salt
4 tablespoons butter, divided
Directions
Preheat your oven to 400°F (200°C). Prepare all ingredients before starting.
In a medium skillet over medium heat, warm ¼ cup of the vegetable oil. Add the thinly sliced onion and cook for about 45 minutes, stirring occasionally, until deeply golden and caramelized. Once done, remove from the skillet and set aside. Carefully wipe out the pan to use later for grilling the sandwiches.
While the onions cook, prepare the mushrooms. Gently remove the stems and scrape out the gills using a spoon. Wipe the caps clean with a damp paper towel if needed. Place the mushrooms cap side down on a parchment-lined baking sheet.
In a small bowl, whisk together the minced garlic, balsamic vinegar, remaining ¼ cup vegetable oil, soy sauce, and dried thyme. Spoon the marinade evenly into the mushroom caps, allowing it to overflow slightly if needed.
Roast the mushrooms in the preheated oven for 20 minutes. Remove and let them cool slightly, then slice them into thick strips.
Lay out the 8 slices of rye bread. Spread the mayonnaise evenly across all slices. Top each slice with a piece of Swiss cheese. Divide the caramelized onions evenly among all slices.
Place the sliced mushrooms onto 4 of the bread slices. Sprinkle the mushrooms evenly with the ¼ teaspoon salt. Close each sandwich with the remaining 4 slices of bread, cheese side down.
Heat the skillet over medium heat and melt 1 tablespoon of butter. Place two sandwiches into the skillet and cook for 3–4 minutes until golden brown. Add another tablespoon of butter, carefully flip the sandwiches, and cook for another 3–4 minutes until the cheese is melted and the bread is crisp. Repeat with the remaining sandwiches, using the remaining butter.
Serve hot.
Servings and timing
Servings: 4 sandwiches
Prep time: 15 minutes
Cook time: 1 hour 10 minutes
Total time: 1 hour 25 minutes
Calories per serving: Approximately 864 kcal
Variations
For a vegan version, substitute plant-based mayonnaise and plant-based Swiss-style cheese, and use oil instead of butter for grilling.
Try using sourdough or classic sandwich bread instead of rye.
Swap Swiss cheese for mozzarella, provolone, or sharp cheddar.
Add sautéed spinach, tomato slices, or even a spoonful of sauerkraut for extra flavor.
For a slightly sweeter marinade, add ½ to 1 tablespoon brown sugar.
Storage/Reheating
Store leftover sandwiches in an airtight container in the refrigerator for up to 5 days.
To reheat, place the sandwich in a skillet over medium heat with a small amount of butter or oil. Cook until the bread becomes crisp again and the cheese melts. Avoid microwaving if possible, as it can make the bread soggy.
For meal prep, prepare the caramelized onions and roasted mushrooms in advance and store them separately in the refrigerator. Assemble and grill the sandwiches fresh when ready to serve.
The cooked mushrooms, caramelized onions, and untoasted rye bread can be frozen separately for 3–6 months. Thaw overnight in the refrigerator before assembling.
FAQs
Can I wash the mushrooms under water?
It’s best to wipe mushrooms clean with a damp paper towel. Washing them directly under water can cause them to absorb moisture and become soggy.
Do I have to remove the mushroom gills?
Removing the gills is recommended because they can add a slightly bitter flavor and darken the sandwich filling.
Can I make this recipe ahead of time?
Yes. Prepare the mushrooms and onions in advance, store them separately, and assemble the sandwiches when ready to cook.
What’s the best bread for a patty melt?
Rye bread is traditional and adds classic flavor, but sourdough or white sandwich bread also work well.
Can I make this recipe dairy-free?
Yes. Use dairy-free cheese and plant-based butter or oil for grilling.
How do I know when the onions are properly caramelized?
They should be deep golden brown, soft, and sweet in flavor. This usually takes about 45 minutes over medium heat.
Can I grill these sandwiches instead of using a skillet?
Yes. A flat-top griddle or panini press works well for even browning.
Why are my mushrooms watery?
Mushrooms naturally release moisture as they cook. Roasting at a high temperature helps evaporate excess liquid.
Can I freeze fully assembled sandwiches?
It’s better to freeze the components separately. Fully assembled sandwiches may become soggy when thawed.
What can I serve with mushroom patty melts?
They pair well with fries, a crisp green salad, coleslaw, or your favorite chips.
Conclusion
This Mushroom Patty Melt transforms simple ingredients into a rich, comforting sandwich packed with savory flavor. The roasted portobellos, sweet caramelized onions, and melted Swiss cheese create a satisfying bite that rivals any traditional diner classic. Whether you enjoy it fresh from the skillet or prepare the components ahead of time, this meatless melt is sure to become a favorite in your kitchen.
A hearty, diner-style meatless patty melt made with roasted balsamic-soy marinated portobello mushrooms, sweet caramelized onions, melty Swiss cheese, and crispy rye bread.
Author:Sophia
Prep Time:15 minutes
Cook Time:1 hour 10 minutes
Total Time:1 hour 25 minutes
Yield:4 sandwiches
Category:Main Course
Method:Baking
Cuisine:American
Diet:Vegetarian
Ingredients
1/2 cup vegetable oil, divided
1 colossal onion (about 565 g or 3 cups thinly sliced)
4 large portobello mushrooms
2 garlic cloves, minced (about 1 tablespoon)
1/4 cup balsamic vinegar
1/8 cup soy sauce
1/2 teaspoon dried thyme
8 slices rye bread
1/3 cup mayonnaise
8 slices Swiss cheese
1/4 teaspoon salt
4 tablespoons butter, divided
Instructions
Preheat oven to 400°F (200°C).
Heat 1/4 cup vegetable oil in a skillet over medium heat. Add sliced onion and cook about 45 minutes, stirring occasionally, until deeply golden and caramelized. Remove and set aside. Wipe out skillet.
Remove mushroom stems and scrape out gills. Place caps on a parchment-lined baking sheet.
Whisk garlic, balsamic vinegar, remaining 1/4 cup oil, soy sauce, and thyme. Spoon marinade into mushroom caps.
Roast mushrooms 20 minutes. Cool slightly, then slice into thick strips.
Spread mayonnaise on all rye slices. Top each with Swiss cheese and divide caramelized onions evenly.
Place sliced mushrooms on 4 bread slices, sprinkle with salt, and top with remaining bread slices, cheese side down.
Heat skillet over medium and melt 1 tablespoon butter. Cook two sandwiches 3–4 minutes until golden. Add another tablespoon butter, flip, and cook 3–4 minutes until cheese melts. Repeat with remaining sandwiches.
Serve hot.
Notes
Caramelize onions slowly for best sweetness and depth.
Roast mushrooms at high heat to reduce excess moisture.
For vegan version, use plant-based mayo, cheese, and oil instead of butter.
Store components separately for best texture when meal prepping.
Reheat in a skillet to keep bread crisp—avoid microwaving if possible.
