Heat a sandwich grill until hot. Set sandwich, butter side down in grill. Butter top side of bread. Place a heavy pan on top of entire sandwich to evenly distribute weight and cook until nicely browned and the cheese is melted, turning once, 3 to 4 minutes on each side.
Transfer the sandwich to a work surface and remove the top slice of bread. Spread the underside of the bread with mustard, and add roasted peppers, if desired.
Replace the top slice of bread, cut the sandwiches into slices, and serve immediately.