A few small changes can boost the flavor even more. Add a splash of balsamic vinegar to the onions near the end of cooking for extra depth. Swap the Swiss cheese for provolone or gruyere if you want a sharper, nuttier taste. A thin layer of caramelized onion jam instead of fresh onions can save time if you’re in a rush. For extra crunch, toast the bread lightly before buttering and grilling it, which gives the crust a firmer bite.
Ingredients
Method
- Heat 1 tablespoon of butter and the olive oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, for about 15 minutes, until soft and golden brown.
- Push the onions to one side of the pan and add the mushrooms. Cook for 8 to 10 minutes, stirring now and then, until browned.
- Stir the onions back in. Add the Worcestershire sauce, garlic powder, salt, and pepper. Cook for another minute, then remove from heat.
- Spread a thin layer of mustard on two bread slices, if using. Butter the outside of each slice.
- Place a slice of cheese on the unbuttered side of two bread slices. Pile the mushroom and onion mixture on top, then add another slice of cheese. Close with the remaining bread, buttered side out.
- Heat a clean skillet over medium-low heat. Cook the sandwiches for 3 to 4 minutes per side, pressing gently with a spatula, until golden and the cheese melts fully.
- Let the sandwiches sit for a minute before cutting in half.