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Mushroom Patty Melt

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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

  1. Preheat oven to 400°F (200°C).
  2. 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.
  3. Remove mushroom stems and scrape out gills. Place caps on a parchment-lined baking sheet.
  4. Whisk garlic, balsamic vinegar, remaining 1/4 cup oil, soy sauce, and thyme. Spoon marinade into mushroom caps.
  5. Roast mushrooms 20 minutes. Cool slightly, then slice into thick strips.
  6. Spread mayonnaise on all rye slices. Top each with Swiss cheese and divide caramelized onions evenly.
  7. Place sliced mushrooms on 4 bread slices, sprinkle with salt, and top with remaining bread slices, cheese side down.
  8. 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.
  9. 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.

Nutrition

Find it online: https://savorymomskitchen.com/mushroom-patty-melt/