01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Slice the tops off each bread loaf and hollow out the centers, leaving about 1/2-inch thick walls and base to form bowls. Reserve the removed bread for another use.
03 - Mix softened butter with minced garlic, parsley, salt, and Parmesan (if using). Spread this mixture evenly inside each bread bowl and on the cut edges.
04 - Place bowls on the prepared baking sheet. Bake for 10–12 minutes until golden and crisp. Set aside.
05 - Cook spaghetti in salted boiling water according to package instructions until al dente. Drain and set aside.
06 - In a large skillet, heat olive oil over medium heat. Add onion and sauté 2–3 minutes until translucent. Add garlic and cook 1 minute more.
07 - Pour in marinara sauce, oregano, pepper, and salt. Simmer for 5–7 minutes.
08 - Add cooked spaghetti to the sauce. Toss well to coat.
09 - Fill each garlic bread bowl with spaghetti mixture. Top with mozzarella (if using). Return bowls to oven and bake an additional 5–7 minutes, until cheese melts and filling is heated through.
10 - Garnish with fresh basil before serving.