Preheat grill to medium-high. Season top sirloin with salt and pepper, and grill to desired doneness. Remove from heat; let rest 5 minutes and slice thinly across the grain.
Butter one side of the bread slices. Place four slices, butter-side down, in skillet over medium heat. Layer steak, peppers, basil, mozzarella and another slice of bread on each.
Butter the tops and toast sandwiches until golden brown, flipping once.