Add 26 ounces frozen Italian meatballs, 24 ounces marinara sauce, and 30 ounces canned diced tomatoes to a 6-quart slow cooker. Stir to combine. Cover and cook on LOW for 6 hours.
When about 20 minutes remain in the meatball cooking time, bring a large pot of salted water to a boil. Cook 1 pound spaghetti according to the package until it's al dente. Drain the water. Add the cooked spaghetti to the slow cooker and stir until well combined.
In a medium bowl, stir together 15 ounces part-skim ricotta cheese, 1/2 cup Parmesan cheese, 1 teaspoon dried basil, 1 teaspoon dried oregano, and 1/2 teaspoon garlic salt. Add scoops of the ricotta mixture over the spaghetti and spread it out evenly over the top.
Sprinkle with 2 cups mozzarella cheese. Replace the lid on the slow cooker and cook for another 20-30 minutes until the mozzarella is melted.
Spoon portions onto dinner plates or pasta bowls, being sure everyone gets some meatballs. Garnish with chopped fresh parsley, if desired.
Notes
A 6-quart slow cooker works best for this recipe and gives you plenty of room to stir in the spaghetti and add the cheese layers.
Cook the spaghetti just to al dente since it will continue cooking during the final 20 to 30 minutes.
For the smoothest, creamiest melt, shred the mozzarella from a block.
Store leftover cheesy spaghetti and meatballs in an airtight container in the refrigerator for up to 4 days. Reheat individual servings in the microwave at 50% power until warmed through.