I accidentally made this too good. Now it’s the only thing anyone wants me to bring to potluck.

Around the 2 1/2-hour mark, quickly lift the lid and check a piece of pasta from the center for doneness. If it’s still a bit firm, replace the lid and continue cooking, checking every 20 to 30 minutes until the mostaccioli is just tender and the cheese on top is fully melted and bubbling at the edges.
For a more browned, bakery-style top, once the pasta is done you can carefully lift out the slow cooker insert (if it’s oven-safe) and place it under a preheated broiler for 3 to 5 minutes, watching closely, until the cheese is deeply golden and starting to blister. Let it sit on a heatproof surface for at least 10 minutes before serving so the casserole can set slightly.
To serve, scoop the baked mostaccioli straight from the slow cooker with a large spoon or spatula, making sure to dig down through the layers to capture plenty of gooey cheese and sauce in each portion. The cheese should stretch as you lift it, and the pasta should be saucy but not soupy.
Variations & Tips
Because this recipe is intentionally stripped down to four ingredients, it’s easy to adapt once you’ve made the original version a few times. For extra richness, stir 1/2 to 1 cup of ricotta into the middle layer with the mozzarella and Parmesan (this will technically add a fifth ingredient, but it turns the dish into something very close to a lazy lasagna). If you like a bit of heat, choose a spicy arrabbiata sauce or add a pinch of crushed red pepper to the jarred marinara before mixing it with the pasta. To keep the four-ingredient rule but add more depth, use a very flavorful sauce—something with roasted garlic or basil—so you’re building complexity without extra chopping. You can also play with the cheeses while still counting them as the same two ingredients: use part-skim mozzarella on the inside for good melt and a richer whole-milk mozzarella on top for extra browning, or swap some of the Parmesan for pecorino Romano for a saltier, sharper finish. If your slow cooker runs hot, check the pasta early at about 2 hours; if it runs cool, give it the full 3 1/2 hours and keep the lid on as much as possible. Leftovers reheat well in the microwave with a splash of water or extra sauce to loosen them, and they freeze nicely in individual portions for easy future potluck emergencies.