Lightly grease a large 5- to 7-quart oval slow cooker, coating the bottom and sides with nonstick spray or a little oil.

Place the uncooked mostaccioli and all of the marinara or spaghetti sauce in a large bowl, and stir until every piece of pasta is coated.

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

Spoon about one-third of the sauced mostaccioli into the bottom of the greased slow cooker and spread it evenly, then sprinkle half of the mozzarella-Parmesan mixture over the pasta.

Add another one-third of the sauced pasta over the cheese layer, then sprinkle the remaining mozzarella-Parmesan mixture over it.

Top with the last third of the sauced mostaccioli, smoothing it gently, then scatter the remaining 1 cup mozzarella and 1/2 cup Parmesan evenly over the top.

Cover the slow cooker and cook on LOW for 2 1/2 to 3 1/2 hours, leaving the lid closed for the first 2 hours if possible.

At around 2 1/2 hours, quickly check a piece of pasta from the center; if it is still too firm, cover and continue cooking, checking every 20 to 30 minutes until the pasta is tender and the cheese is melted and bubbling.

For a deeper golden top, place the oven-safe slow cooker insert under a preheated broiler for 3 to 5 minutes, watching it closely.

Let the mostaccioli rest for at least 10 minutes, then scoop through all the layers so each serving includes pasta, sauce, and melted cheese.