Lightly grease a large oval slow cooker with nonstick spray.

In a large bowl, combine the uncooked mostaccioli and jarred marinara sauce, stirring well.

In a separate bowl, mix 2 cups of mozzarella cheese with 1/2 cup of Parmesan cheese.

Spoon one-third of the sauced mostaccioli into the slow cooker and spread it evenly. Sprinkle half of the cheese mixture on top.

Repeat the process with another one-third of the mostaccioli and the remaining cheese mixture.

Finally, top with the last one-third of the mostaccioli.

Combine the remaining mozzarella and Parmesan cheese, then sprinkle it over the top.

Cover and cook on LOW for 2 1/2 to 3 1/2 hours. Check the doneness around the 2 1/2-hour mark, cooking until the pasta is tender and the cheese is bubbling.

For a browned top, you can use a broiler for an additional 3 to 5 minutes if desired.

Let sit before serving. Serve hot, paired with a side salad and garlic bread, and optionally sprinkle with fresh herbs like basil or parsley.