Pepperoni pizza grilled cheese sandwiches are epic! This is the best grilled cheese sandwich recipe I've ever made. Pizza and grilled cheese combined!
Heat olive oil in a pan over medium heat. Once hot cook bell peppers, mushrooms and onions until softened 3 - 5 minutes. Remove from heat.
Use one tablespoon of butter to butter one side of each slice of bread. Sprinkle with garlic salt.
To make one sandwich place 4 slices of pepperoni on a slice of bread. Place 2 slices of mozzarella on top. Spoon bell pepper and onion mixture over the top. Top with other slice of bread. Repeat with remaining ingredients.
Heat remaining tablespoon of butter in a skillet. Once melted, begin grilling sandwiches. Cook until cheese is melted and bread is golden, 3 - 5 minutes. Serve hot.