1. Bring a large pot of well-salted water to a boil for the spaghetti but do not add the pasta yet.

2. In a large bowl, stir together the breadcrumbs and milk and let sit for 1 minute to form a thick paste.

3. Add the ground beef, mild Italian sausage, 0.25 cup of the Parmesan, the egg, 2 tablespoons of the chopped parsley, 2 minced garlic cloves, Italian seasoning, 0.5 teaspoon of the kosher salt, and the black pepper to the bowl.

4. Gently mix the meatball mixture with clean hands or a fork until everything is just combined, being careful not to overwork it.

5. Roll the mixture into meatballs about 1.5 inches in diameter and place them on a parchment-lined tray or plate.

6. Heat the olive oil in a large deep skillet or wide pot over medium-high heat.

7. Add the meatballs in a single layer (in batches if needed) and brown them on all sides, turning carefully with tongs; they do not need to be cooked through yet.

8. Transfer the browned meatballs to a plate and reduce the heat to medium.

9. In the same pot, add the chopped onion and cook, stirring often, until softened and lightly golden, about 5 minutes.

10. Stir in the remaining minced garlic clove and cook for 30 seconds until fragrant.

11. Add the crushed tomatoes, tomato paste, granulated sugar, remaining 0.5 teaspoon kosher salt, a pinch of black pepper, crushed red pepper flakes if using, and the bay leaf.

12. Stir well to combine, scraping up any browned bits from the bottom of the pot, and add a splash of water if the sauce looks very thick.

13. Gently return the browned meatballs to the pot, nestling them into the sauce in a single layer.

14. Bring the sauce to a gentle simmer, then reduce the heat to low, cover partially, and cook for 20–25 minutes, turning the meatballs once or twice, until they are cooked through and tender.

15. While the sauce simmers, cook the spaghetti in the boiling salted water according to package directions until al dente.

16. Reserve about 1 cup of the starchy pasta water, then drain the spaghetti.

17. Taste the sauce and adjust seasoning with additional salt, pepper, or a pinch more sugar if needed; remove and discard the bay leaf.

18. Toss the cooked spaghetti with a few ladles of the tomato sauce, adding a splash of reserved pasta water if needed so it coats the noodles.

19. Divide the sauced spaghetti among plates or into a large serving bowl and top with the meatballs and extra sauce.

20. Sprinkle with the remaining chopped parsley and extra Parmesan cheese before serving hot.