Brown beef & sausage in a large skillet over medium heat. Add onion while browning meat. When brown, drain off any excess fat and return to the skillet. Add tomato sauce, paste, Italian seasoning, garlic, salt & pepper. Mix well. Simmer for 5-8 minutes. Add cooked pasta. Mix well again and remove from heat.
While the meat sauce is simmering, carve out the middle of the loaf. Remove any remaining bread to create a large hollow. Brush insides of the hollow with butter. Sprinkle it with garlic powder. Toast in the oven for 3-5 minutes.
When the bread is toasted, spoon the prepared spaghetti & meat sauce into the hollow, filling it completely. (You will have spaghetti & meat sauce leftover). Top with the cheeses.
Bake at 375° for 5-10 minutes or until the cheese is melted and golden. Remove from the oven, garnish with parsley, and gently cut with a bread knife to serve.