Sauté the Vegetables: In a large skillet, melt the butter over medium heat. Add the sliced mushrooms and onions, and sauté until they are softened and golden brown, about 8-10 minutes.
Add Garlic: Stir in the minced garlic and cook for another 1-2 minutes until fragrant.
Season: Season with salt and pepper to taste. Remove from heat and set aside.
Assemble the Panini
Preheat the Panini Press: If you don’t have a panini press, you can use a grill pan or a regular skillet with a heavy lid.
Build Your Sandwich: On a clean surface, lay out your bread slices. Place a slice of cheese on each piece of bread. Spoon the mushroom and onion mixture onto one side, spreading it out evenly. Top with the other slice of bread, cheese-side down, to form a sandwich.
Grill the Panini
Brush with Olive Oil: Lightly brush the outside of each sandwich with olive oil. This helps to achieve a golden, crispy crust.
Cook the Panini: Place the sandwiches in the preheated panini press and cook until the bread is golden and the cheese is melted, about 3-5 minutes. If using a skillet, cook on one side until golden brown, then flip and cook the other side, pressing down with a spatula or heavy lid.
Slice and Serve: Remove the panini from the press or skillet. Let them cool for a minute, then slice in half and serve hot.