This easy lunch sandwich is melty, cheesy, gooey, with a wonderful earthy umami flavor thanks to sauteed mushrooms and onions. Read the notes below the recipe for make-ahead tips and recipe variations!
3cupsbaby bella mushroomscleaned and sliced (about a 10-16 oz package)
1yellow onionthinly sliced into half moons
1teaspoonsaltdivided, or to taste
¼ - ½teaspoonground black pepperto taste
3tablespoonsbutterdivide (or use olive oil)
2tablespoonsfresh parsley leavesfinely minced
4slicesbread(sliced white bread, such as brioche, sourdough, or Texas toast, but whole wheat is also a great option!)
2-4slicesmozzarella cheese(or use any combination of mozzarella, provolone, white cheddar, gouda... or try smoked versions of each of these cheeses!)
Instructions
Heat a large pan over medium heat. Add 2 tablespoons of butter (or olive oil), sliced onion, half the salt, and pepper. Sauté for about 20 minutes and let the onions caramelize without burning. If needed add a few tablespoons of water to prevent burning.
Remove the onions to a bowl, and add the mushrooms and the rest of the salt. Turn up the heat and cook over medium-high heat for 5-7 minutes or until cooked down to your liking, stirring constantly.
Add the mushrooms to the bowl with the onions. Add the parsley, and stir everything together.
Assemble your sandwiches: place 1 slice of cheese on your bread. Top with half the mushroom mixture. If using more cheese, top the mushroom mixture with another slice, and top with the other bread.
Use tongs and a damp paper towel to wipe down the skillet. Melt the butter in the pan and cook the sandwiches until the bread is golden brown and the cheese is melted.
Notes
If cooking both sandwiches in 1 large pan, melt about ½ tablespoon of butter in the pan, cook on one side, then add the rest of the butter and cook on the other side. If using two smaller pans, use about ¼ tablespoon butter for each side of the sandwich.
Try a pinch of cayenne pepper for some spice, or different herbs, such as fresh thyme or torn basil leaves.
This mushroom + onion combination works great as a quesadilla too.
Instead of cooking the sandwich in butter, spread some mayo on the outsides before assembling and use the fat from the mayo for cooking.
Prep ahead: You can sauté the onions and mushrooms in advance and just keep the mixture in the fridge for up to 3-4 days. The only thing is that the cheese will take longer to melt. You can heat up the mushroom mixture before assembling your sandwich or cover your pan with a lid to keep the heat in.