Preheat oven to 375°F (190°C). Line a baking sheet with foil.
In a large skillet over medium heat, add olive oil and sauté onions for 2 minutes. Add garlic and cook 1 more minute.
Add ground beef and sausage. Cook until browned, breaking up meat as it cooks. Drain excess fat if needed.
Stir in marinara sauce, red pepper flakes, salt, and pepper. Simmer for 3–4 minutes until thick.
Slice hoagie rolls down the center, keeping one side intact. Fill each with the meat mixture and top generously with shredded mozzarella.
Place on the baking sheet and bake for 10–12 minutes, or until cheese is bubbly and golden.
Let rest 5 minutes. Top with banana peppers if desired and serve hot.
