Pour the sauce-and-water mixture evenly over the dry rotini in the glass dish, nudging the pasta with a spoon so most of it is submerged in the liquid.
With clean hands, scatter the frozen meatballs evenly over the top of the sauced, dry rotini. Don’t worry if some meatballs sink a little; they’ll settle in as everything bakes.
Cover the glass baking dish tightly with foil, making sure the edges are well sealed so the steam stays inside and cooks the pasta.
Bake, covered, in the preheated oven for 35–40 minutes, until the pasta is tender when you peek under the foil and test a piece with a fork.
Carefully remove the foil (watch out for hot steam), sprinkle the shredded mozzarella evenly over the top of the meatballs and pasta, and return the dish to the oven uncovered.
Bake for another 10–15 minutes, until the cheese is melted, bubbly, and lightly golden in spots and the sauce is thickened around the edges.
Let the meatball pasta bake rest on the counter for 5–10 minutes before serving so it can set up a bit. Spoon into bowls and enjoy while it’s good and hot.
If your crowd likes things extra saucy, add an extra 1/2 to 1 cup of water along with the sauce; just know it may need a few more minutes in the oven to thicken. You can swap the marinara for any tomato-based pasta sauce you like—herb, garlic, or even a chunky vegetable version all work nicely. For a cheesier top, mix 1/2 cup grated Parmesan with the mozzarella before sprinkling it over the casserole. If you prefer a little zip, scatter red pepper flakes or a few shakes of Italian seasoning over the meatballs before baking. Whole wheat rotini can be used, but it may need an extra 5–10 minutes under the foil and a splash more water if it looks dry. To sneak in some vegetables, tuck a layer of frozen peas or small broccoli florets under the meatballs before covering with foil; they’ll steam along with the pasta. Leftovers reheat well in the oven, covered with foil, at 350°F until warmed through, and a small splash of water in the dish will help bring the sauce back to life.
