Line a baking sheet with aluminum foil and place meatballs in a single layer on the sheet. Bake in a preheated 375-degree F oven for 15 to 20 minutes, turning once.
Mix together the marinara sauce and baby spinach in the bottom of a 13-x9-inch baking dish. Place the cooked meatballs in the dish in the sauce. Top the meatballs with teaspoonfuls of ricotta cheese. Sprinkle the shredded mozzarella on top.
Bake for 30 minutes or until the casserole bubbles. If you’d like the cheese on the top browned, place the dish under the broiler for an additional 3 to 4 minutes, until the cheese begins to brown.