Cook the Filling: In a skillet over medium heat, cook beef, onion, and peppers until beef is done and vegetables are soft. Season with salt and pepper.
Add Butter Topping: Mix melted butter with garlic powder and brush over tops.
Bake: Cover loosely with foil and bake at 180°C (350°F) for 15 min.
Classic version: green bell peppers + provolone
Creamy version: stir in cream cheese
Sweet Pepper version: use red & yellow bell peppers
Garlic Butter version: brush rolls with garlic butter
Mini Mozzarella version: swap provolone for mozzarella
Assemble up to 6 hours ahead and refrigerate. Bake just before serving.
Leftovers store in the fridge for up to 2 days. Reheat covered to maintain softness.