# The Ingredients You'll Need:
→ Vegetables
01 - 1 cup sliced cremini or button mushrooms (about 3.5 oz)
02 - 1 small yellow onion, thinly sliced (optional)
03 - 1 garlic clove, minced
→ Dairy & Cheese
04 - 4 slices Swiss cheese
05 - 2 tablespoons unsalted butter, divided
→ Bread
06 - 4 slices hearty sandwich bread, such as sourdough or country loaf
→ Seasonings
07 - Salt, to taste
08 - Black pepper, to taste
09 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme (optional)
# Step-by-Step Instructions:
01 - Melt 1 tablespoon butter in a skillet over medium heat. Add sliced mushrooms and onion if using; sauté for 5 to 7 minutes until softened and browned. Stir in minced garlic and thyme; cook an additional minute. Season with salt and pepper. Remove mixture from skillet and set aside.
02 - Arrange bread slices on a work surface. Place 1 slice Swiss cheese on each piece of bread. Evenly distribute mushroom mixture onto 2 slices. Top each with a second slice of cheese and cover with remaining bread, cheese-side down, to form sandwiches.
03 - Melt remaining tablespoon butter in the skillet over medium-low heat. Add sandwiches and cook for 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden brown and cheese is melted.
04 - Remove sandwiches from skillet. Slice diagonally and serve immediately while hot.