Preheat the oven to 375°F (190°C). Line a baking sheet with aluminum foil for easy cleanup.

Arrange the meatballs on the prepared sheet in a single layer. Bake for 15–20 minutes, turning once halfway through, until browned and cooked through.

Spread the marinara sauce in the bottom of a 9x13-inch baking dish or a large round pan. Stir the packed baby spinach into the sauce to wilt.

Transfer the cooked meatballs into the sauced spinach, pressing each down slightly.

Spoon teaspoonfuls of ricotta cheese over the tops of the meatballs, distributing them evenly.

Sprinkle the shredded mozzarella across the entire dish so the meatballs and sauce are well covered.

Bake at 375°F for 30 minutes, until the casserole is bubbly and cheese is melted. Optionally, place under the broiler for 3–4 minutes for a browned top, watching carefully.

Let rest for 5 minutes before serving.