Heat a non-stick skillet over medium heat and melt 1 tablespoon of unsalted butter. Add sliced mushrooms and sauté for about 5 minutes, until browned and tender.

Add thinly sliced onions and minced garlic to the skillet. Cook for additional 4 to 5 minutes, stirring frequently, until onions are soft. Season with salt and pepper.

In a small bowl, mix together mayonnaise and Dijon mustard until smooth. Spread the mixture generously on one side of each slice of bread.

Layer a slice of Swiss cheese on the unspread side of one slice of bread, followed by the sautéed mushroom and onion mixture, then another slice of Swiss cheese on top. Place the second slice of bread on top, mayo-mustard side facing in.

Melt the remaining tablespoon of butter in the same skillet. Place your sandwich in the skillet and grill for about 3 to 4 minutes until the bread is golden brown.

Flip the sandwich carefully with a spatula and cook for another 3 to 4 minutes until browned and cheese melts.

Remove the Swiss Mushroom Melt from the skillet and let it rest for 1 minute. Slice in half and serve immediately.