In a large pot or Dutch oven, heat a tablespoon of olive oil over medium-high heat. Add the ground beef (or Italian sausage) and cook, breaking it up with a spoon, until browned. Drain off any excess grease.

Add the chopped onion to the pot and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute until fragrant.

Stir in the crushed tomatoes, tomato sauce, tomato paste, oregano, basil, salt, and pepper. Bring the sauce to a simmer, then reduce the heat to low, cover, and let it simmer for at least 30 minutes, or up to an hour, stirring occasionally. Stir in the fresh parsley, if using, just before serving.

While the sauce is simmering, bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente.

Drain the spaghetti well and add it directly to the pot of spaghetti sauce. Toss to coat the spaghetti evenly with the sauce.

Preheat your oven to 350°F (175°C).

Using a serrated knife, carefully cut a circle out of the top of each bread loaf, leaving about a 1-inch border. Remove the bread from the center of each loaf, creating a bowl shape.

In a small bowl, combine the melted butter, minced garlic, and chopped parsley.

Brush the inside and outside of each bread bowl with the garlic butter. Sprinkle the inside of each bowl with grated Parmesan cheese.

Place the prepared bread bowls on a baking sheet and bake for 10-15 minutes, or until they are golden brown and slightly crispy.

Remove the bread bowls from the oven and carefully fill each one with the spaghetti and sauce mixture.