01 - Heat a skillet over medium heat. Add sliced mushrooms and onions, sautéing until caramelized and golden brown. Remove mixture from pan and allow to cool slightly.
02 - Lightly butter the outside surface of each bread slice. On the unbuttered side of two slices, evenly distribute shredded Swiss cheese and the sautéed mushroom-onion mixture. Close with remaining bread, keeping buttered sides facing outward.
03 - Place assembled sandwiches in a preheated skillet over medium heat. Grill for 3 to 4 minutes per side until bread is golden and crisp, and the cheese has melted. Slice and serve immediately.