Combine the bread pieces and 2/3 cup cold water in a bowl, let them sit for 5 minutes, then mash the mixture with a fork. The bread should look fully softened and pasty, which is what helps the meatballs stay tender.

Add the ground beef, sausage, parmesan, 4 cloves garlic, sea salt, black pepper, egg, and mashed bread to a large bowl or the bowl of a stand mixer fitted with the paddle, then mix just until combined. Stop as soon as the mixture looks evenly blended so the meatballs stay light instead of tight and dense.

Shape the meat mixture into 1 1/2-inch meatballs, about the size of a flat ice cream scoop, then roll them in flour and shake off the extra. Keeping them close in size helps them brown evenly and finish cooking at the same pace.

Heat a deep large heavy skillet or Dutch oven over medium heat with about 3 Tbsp oil, then brown the meatballs in 2 batches for about 6 minutes total, turning every 2 minutes to color all sides. When the pan is ready, the meatballs should sizzle right away and smell deeply savory as a golden crust forms.

Transfer the browned meatballs to a plate and leave the pan on the stove. They do not need to be cooked through yet because they will finish gently in the sauce.

Add a little more oil to the same skillet if needed, then cook the chopped onion over medium heat for 5 minutes, stirring often, until soft and golden. The onions should move from sharp and pale to sweet-smelling and lightly golden, which sets up a fuller sauce.

Stir in the remaining 4 cloves garlic and cook for 1 to 2 minutes. Take the next step when the garlic smells fragrant and the onion looks glossy, not browned.

Add the crushed tomatoes and bay leaves, then bring the sauce to a light boil while stirring. Look for steady bubbling across the surface rather than a few lazy bubbles around the edges.

Return the meatballs to the sauce, partially cover, and keep the pan at a gentle simmer for 30 minutes, turning the meatballs occasionally. By the end, the sauce will be a little thicker and darker, and the meatballs should feel tender all the way through.

Stir in the basil a few minutes before the sauce is done, then season with salt and pepper to taste while the spaghetti cooks in salted water according to the package directions until al dente. The basil should smell fresh and bright, and the pasta should still have a little bite so it holds the sauce well.

Drain the spaghetti and return it to the empty pot, then add the sauce and meatballs and toss gently to combine before serving hot with more parmesan and basil if desired. If the mixture feels thicker than you like, loosen it with a splash of reserved pasta water until it coats the spaghetti evenly.