The meatballs. Add the onion, garlic and parsley to your food processor. Pulse until everything is finely minced, scraping down the sides if needed.
Add the mixture to a large bowl along with ground beef, Italian sausage, egg, Parmesan, Dijon mustard, bread crumbs, marinara sauce, Italian seasoning, salt and pepper. Mix JUST until combined. Don't overmix otherwise the meatballs will be dense.
Roll into 1 1/2-2-inch meatballs ( about 2 tablespoons). Dredge meatballs in flour, shaking off excess.
Heat oil in a large skillet over medium high heat. Add the meatballs and saute until browned on all sides, about 5-6 minutes. You will need to saute them in batches not to overcrowd the pan. Transfer them to a plate. Set aside.
The sauce. In the same skillet over medium heat, add the chopped onion and bay leaves and cook until the onion is translucent, about 4 minutes.
Add the garlic and cook further until fragrant, about 1 minute.
Stir in the crushed tomatoes and bring to a gentle simmer.
Add the meatballs, partially cover with the lid and allow to simmer for 25-30 minutes. The sauce will thicken slightly. Season with salt and pepper to your liking. Garnish with the chopped basil leaves.
Meanwhile, cook spaghetti according to the package instructions. Drain.
Ladle the sauce with meatballs over the spaghetti and toss gently using kitchen tongs.
Garnish with fresh basil leaves and grated parmesan cheese. Enjoy!