Coat a vegetable tray with butter. Place bell peppers, onions, mushrooms and onions, and cook on the grill for 3-5 minutes, or until vegetables are softened.
Butter one side of each slice of bread. Sprinkle with garlic salt
To make one sandwich, place 4 slices of pepperoni on the unbuttered side of a slice of bread followed by 2 slices of mozzarella cheese. Spoon vegetable mixture over top. Top with a slice of bread, buttered side up. Repeat with remaining ingredients for additional sandwiches
Place sandwiches on grill and cook until cheese is melted and bread is golden, about 2-4 minutes per side
If you would rather use a pan, heat remaining tablespoon of butter in a cast iron pan on the grill. Once melted, begin grilling the sandwiches. Cook until cheese is melted and bread is golden, about 3-5 minutes per side.