Heat a pan with a little oil until hot. Add the sliced onions and sauté until fragrant.

Once the onions are translucent, add the mushroom stems and sauté until cooked. Season with salt and pepper.

Spread butter on both sides of the bread slices.

Heat a pan until hot. Place the bread slices in the pan, then evenly distribute mozzarella cheese over the bread. Add the sautéed mushrooms and another layer of mozzarella cheese. Top with another slice of bread.

Grill both sides of the sandwich over low heat until the desired color is achieved. Remove from heat, slice, and serve.

Arrange and garnish the dish as desired.