Slice the Italian loaf lengthwise through the middle without cutting completely through creating a hinged bread boat. Carefully hollow out some of the inner bread leaving about a 1/2 inch border all around to form a sturdy vessel that will contain all the delicious filling.
Cook the vegetables:
Melt garlic butter in a large skillet over medium heat until fragrant. Add sliced onions and bell peppers spreading them evenly across the pan. Cook for 7 to 8 minutes stirring occasionally until they become soft and the onions turn translucent with light caramelization around the edges.
Brown the beef:
Push vegetables to one side of the pan and add the thinly sliced beef to the cleared area. Season generously with salt and pepper. Cook for 3 to 4 minutes breaking the meat apart with a spatula until just browned do not overcook as it will continue cooking in the oven.
Assemble the loaf:
Spread a thin layer of additional garlic butter inside both sides of the hollowed bread. This creates a moisture barrier and adds extra flavor. Spoon the beef and vegetable mixture evenly into the bread cavity packing it down slightly to create a level surface.
Add cheese and bake:
Cover the filling completely with shredded provolone creating an even blanket of cheese. Place the stuffed loaf on a baking sheet and bake in the preheated oven for 15 to 20 minutes until the bread develops a golden crust and the cheese melts completely with light browning on top.
