Preheat oven to 400°F (205°C).
Make the sauce: Heat 2 tbsp olive oil in a heavy skillet or Dutch oven over medium heat. Sauté the diced onion with a pinch of salt until translucent (5–7 minutes). Add 4–5 minced garlic cloves and cook 30–60 seconds until fragrant.
If using meat: add 1 lb ground beef or sausage, break up and brown thoroughly; drain excess fat if necessary, leaving about 1 tbsp for flavor.
Stir in 2 tbsp tomato paste and cook 1–2 minutes to deepen flavor. Add the crushed tomatoes, 1 tsp sugar (if using), dried oregano, dried basil, and red pepper flakes. Bring to a gentle simmer, reduce heat to low, and simmer 15–20 minutes until slightly reduced. Finish with ½ cup grated Parmesan and chopped parsley; adjust salt and pepper.
Cook the spaghetti: Bring a large pot of salted water to a boil. Cook spaghetti about 1 minute less than package directions (it will finish in the sauce). Reserve 1 cup pasta water, drain spaghetti, then toss spaghetti into the sauce with a splash or two of reserved pasta water to finish cooking and help the sauce cling to the noodles. Keep warm.
Prepare garlic butter and bowls: While sauce simmers, slice the top ⅓ off each boule and hollow out the interior, leaving about ¾–1" of wall. Save the removed crumb for dipping or toasts. Mix the softened butter with 3–4 minced garlic cloves, 2 tbsp chopped parsley, oregano (if using), lemon zest (optional), and a pinch of salt. Spread garlic butter generously inside each hollowed bowl and on the removed lids. Brush outer crust lightly with olive oil. Sprinkle 1–2 tbsp shredded mozzarella inside each bowl to create a cheesy seal.
Bake the bread bowls: Place bowls on a parchment-lined baking sheet and bake 12–15 minutes until golden and fragrant. For extra crispness, broil 1–2 minutes (watch closely).
Assemble: Spoon a generous mound of saucy spaghetti into each hot garlic bread bowl. Top with a handful of shredded mozzarella and a dusting of Parmesan. Broil 1–3 minutes just until cheese melts and bubbles (watch carefully).
Finish & serve: Garnish with chopped parsley and cracked black pepper. Serve immediately with the bread lids or reserved interior crumbs for scooping.