Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch casserole dish.
Scatter 12 oz dry pasta evenly over the bottom of the prepared dish. Do not precook.
Pour 24 oz brothy pasta sauce over the pasta, using a spoon to ensure even coverage and no dry spots.
Tuck 16 oz frozen meatballs down into the saucy pasta in an even layer.
Sprinkle 4 cups chopped spinach over the top. It will look like a lot but will wilt down.
Cover tightly with aluminum foil to trap steam. Bake on middle rack for 35-40 minutes until pasta is tender.
Remove foil carefully. Stir gently from edges to center. If dry, add a few tablespoons of water or broth.
Return uncovered dish to oven and bake 5-10 more minutes until top is lightly golden and casserole is bubbling.
Let rest 5-10 minutes before serving to allow pasta to settle and absorb remaining liquid.