Step 1

Prepare meatballs according to package directions.

Step 2

Meanwhile, heat oil in a large, broiler-safe skillet over medium heat. Add onion and cook, stirring occasionally, until tender, about 2 minutes. Add garlic and cook, stirring, for 30 seconds. Add spinach and cook, stirring occasionally, until thawed, 6 to 8 minutes. Sprinkle with flour and stir to combine. Add milk and cook, stirring occasionally, until thickened, about 2 minutes. Stir in pepper and nutmeg.

Step 3

Nestle the cooked meatballs into the creamed spinach. Drizzle with marinara and scatter mozzarella on top.

Step 4

Position rack in upper third of oven and turn broiler to high. Broil the casserole until the cheese is browning in spots, 3 to 4 minutes.