01 - Preheat your oven to 375°F. Line a baking sheet with parchment paper to prevent sticking and ensure easy cleanup.
02 - Using a sharp bread knife, slice the tops off each bread loaf. Carefully hollow out the centers, leaving approximately 1/2-inch thick walls and base to create sturdy bowls. Set aside the removed bread for another use.
03 - In a small mixing bowl, combine softened butter with minced garlic, chopped parsley, salt, and Parmesan cheese if using. Mix thoroughly until well incorporated.
04 - Spread the garlic butter mixture evenly inside each bread bowl and along the cut edges. Place bowls on the prepared baking sheet. Bake for 10-12 minutes until golden brown and crispy. Remove and set aside.
05 - Bring a large pot of salted water to boil. Cook spaghetti according to package directions until al dente. Drain thoroughly and set aside.
06 - In a large skillet, heat olive oil over medium heat. Add chopped onion and sauté for 2-3 minutes until translucent. Add minced garlic and cook for 1 additional minute until fragrant.
07 - Pour marinara sauce into the skillet. Add dried oregano, black pepper, and salt. Reduce heat to low and simmer for 5-7 minutes, stirring occasionally.
08 - Add cooked spaghetti to the sauce. Using tongs or a pasta fork, toss thoroughly to coat the pasta evenly with sauce.
09 - Fill each garlic bread bowl with the spaghetti mixture. Top with mozzarella cheese if desired. Return to oven and bake for 5-7 minutes until cheese is melted and filling is heated through.
10 - Remove from oven and let cool for 2-3 minutes. Garnish with fresh basil leaves before serving hot.