Heat a pan and add oil. Sauté the onions until fragrant.

Continue to sauté the onions until translucent, then add the mushroom stems and cook until done. Season with salt and pepper.

Spread butter on both sides of the bread slices.

Heat a pan until hot. Place a slice of bread in the pan, add mozzarella cheese, followed by the sautéed mushrooms, more mozzarella cheese, and top with another slice of bread.

Grill the sandwich on low heat on both sides until it reaches the desired color. Then, cut and serve.

Garnish and plate as desired.