Check and clean the burn pot. Before placing anything on the grill, generously oil the cooking grids, using a cloth and vegetable oil.
DO NOT use a spray oil, as this does have the potential to flame.
Preheat grill to 400°F.
1. 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.
2. Butter one side of each slice of bread. Sprinkle with garlic salt.
3. 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.
4. Place sandwiches on grill and cook until cheese is melted and bread is golden, about 2-4 minutes per side.
5. 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.