Preheat the oven to 375°F (190°C).
Heat olive oil in a skillet over medium heat, add onions and bell peppers, sauté until softened.
Add garlic and diced flank steak to the skillet. Season with salt and pepper, cooking until steak is browned.
Introduce the shrimp to the skillet, cooking until they turn pink and opaque.
Remove from heat, stir in mozzarella cheese until melted.
Prepare the French loaf by slicing off the top and hollowing out the interior.
Stuff the hollowed bread with the steak and shrimp mixture, packing it gently.
Replace the top of the bread and wrap it in aluminum foil.
Bake for 25-30 minutes, then unwrap and bake for an additional 10-15 minutes until golden.
Remove from the oven and let cool slightly before slicing and serving.