In a skillet over medium heat, add the ground Italian sausage and finely chopped yellow onion. Cook for 6–8 minutes, stirring occasionally until the sausage is browned and the onion is translucent.

Add the minced garlic and sauté for an additional minute, allowing the garlic to become fragrant.

Stir in the marinara and tomato sauce, along with ½ cup of water or low-sodium chicken broth. Add the dried Italian seasoning, salt, black pepper, and optional crushed red pepper flakes.

Let the sauce simmer for 3–5 minutes, allowing the flavors to meld together.

Lightly grease the insides of your slow cooker. Spread a thin layer of the meat sauce mixture on the bottom.

Begin with ⅓ of the uncooked ziti pasta, then layer ⅓ of the meat sauce, followed by ⅓ of the ricotta cheese, and ⅓ each of the mozzarella and Parmesan cheeses. Repeat until all ingredients are used.

Cover the slow cooker and set it to cook on low for 3 ½ to 4 hours.

Once cooked, let your Crock Pot Baked Ziti sit uncovered for 10–15 minutes.

Before serving, sprinkle with freshly chopped basil or parsley.