In a large skillet set over medium-high heat, heat oil. Add ground beef, sausage, and onion and cook 5-6 minutes, until beef is cooked through, crumbling with a spoon as you cook. Drain any excess grease and return it to the skillet.
2 teaspoons olive oil, 1 pound ground beef, 1 pound ground Italian sausage, 1 cup onion
Add the garlic and cook for 1 minute, until fragrant. Stir in the marinara sauce, pouring the water into the jar and swishing it around to remove any excess sauce, then add that to the skillet. Add diced tomatoes, Italian seasoning, salt, pepper, and sugar, stirring to combine.
3 cloves garlic, 24 ounces marinara sauce, 14 ounces diced tomatoes, 2 teaspoons Italian seasoning, ½ teaspoon kosher salt, ¼ teaspoon black pepper, Pinch granulated sugar, 1 cup water
Transfer the mixture to the crock of a 6-quart slow cooker.
Top with broken spaghetti and stir together.
8 ounces spaghetti noodles
Top with mozzarella, cover, and cook on low for 4 hours.
1 ½ cups shredded mozzarella
Serve garnished with grated Parmesan and fresh parsley or basil.