1. Preheat the oven to 375°F.
2. In a large saucepan, boil the spaghetti until al dente. Remove from heat and drain.
3. While the pasta is cooking, brown the beef and sausage in a large skillet over medium heat. Add diced onions while browning the meat. Once browned, drain any excess fat and return to the skillet. Add tomato sauce, tomato paste, Italian seasoning, minced garlic, salt, and black pepper. Mix well and simmer for 5-8 minutes. Add the cooked pasta to the sauce and mix well. Remove from heat.
4. While the meat sauce is simmering, cut the Italian bread loaf in half using a bread knife. Carve out the middle of each half to create a large hollow. Brush the insides and sides of the halves with butter and sprinkle with garlic powder. Toast the bread in the oven.
5. Once the bread is toasted, spoon the prepared spaghetti and meat sauce into the hollow of each half, filling them completely. Top one loaf with half of the divided Parmesan and Mozzarella cheese, and the other loaf with the other half.
6. Bake at 375°F for 15-20 minutes or until the cheese is melted and golden. Remove from the oven and gently cut with a bread knife to serve.
