Ingredients
Method
- Preheat the oven to 400°F. Gather and prepare all ingredients.
- In a medium skillet over medium heat, warm ¼ cup vegetable oil. Add the sliced onions and cook, stirring occasionally, for about 45 minutes until caramelized and golden. Remove and set aside.
- While the onions are cooking, remove the stems and gills from the mushrooms and place them, cap side down, on a parchment-lined sheet tray.
- In a small bowl, whisk together the garlic, balsamic vinegar, remaining ¼ cup vegetable oil, soy sauce, and thyme. Carefully pour this mixture over the mushroom caps. It’s okay if it overflows.
- Roast the mushrooms in the oven for 20 minutes, then remove and let them cool. Once cooled, slice the mushrooms.
- To assemble the patty melts, lay out the rye bread and spread mayo evenly over all 8 slices. Top each slice with Swiss cheese and caramelized onions. Add the sliced portobello mushrooms to 4 slices of bread and sprinkle with salt, then close with the remaining 4 slices to form sandwiches.
- Melt 1 tablespoon butter in the skillet over medium heat. Place two sandwiches in the skillet. Toast for 3–4 minutes, then add another tablespoon of butter, carefully flip, and cook for 3-4 more minutes. Repeat with the remaining two sandwiches and butter.
- Serve and enjoy!
Nutrition
Notes
For a vegan version, swap the mayo, cheese, and butter with plant-based alternatives. You can also use oil to toast the sandwiches instead of butter.