Cover the slow cooker with its lid and cook on LOW for 3 1/2 to 4 1/2 hours, or until the spaghetti is tender, most of the liquid has been absorbed, and the edges are deeply browned and pulling slightly away from the ceramic sides. Avoid lifting the lid during the first 3 hours so the heat and steam can build and set the pie.
Once the center is set and the top is glossy and bubbling, turn the slow cooker to WARM and let the spaghetti pie rest, covered, for 15 to 20 minutes. This brief rest helps the starches firm up so you can cut neater wedges.
Run a thin spatula or butter knife around the edge of the crock to loosen the caramelized, cheesy perimeter. Cut the spaghetti pie into wedges or large scoops and serve directly from the slow cooker, making sure everyone gets some of the golden, umami-rich crust from the sides.
For a meatier version, tuck 1 to 1 1/2 cups of cooked and crumbled Italian sausage, ground beef, or leftover meatballs (broken into small pieces) in with the dry spaghetti before adding the liquid; keep the total ingredient count to five by using a meat sauce in place of plain marinara. To lean more vegetarian, use vegetable broth and a meatless marinara, then add a handful of olives or chopped sun-dried tomatoes for depth. If you like a bit of heat, choose an arrabbiata-style sauce or add red pepper flakes at the table so you don’t alter the core five-ingredient base. For a richer, more custardy texture, you can whisk one beaten egg into the sauce-and-broth mixture before pouring it over the pasta, but be aware this technically adds an extra ingredient beyond the promised five. Different cheeses also work: swap part of the mozzarella for provolone or an Italian blend, or increase the Parmesan for a saltier, sharper crust. If your slow cooker runs hot, start checking at the 3-hour mark; for more browning on top, you can briefly transfer the ceramic insert (if oven-safe) to a 400°F oven for 5 to 10 minutes, watching closely. Leftovers can be chilled, cut into slabs, and pan-fried in a little oil the next day for crisp-edged spaghetti pie squares.
