Preheat your oven to 400°F (200°C) and prepare your casserole dish by greasing it.

In a large mixing bowl, combine pasta, frozen meatballs, bell peppers, diced onion, and spinach. Pour in the marinara sauce and stir until well-coated.

Transfer the mixture to the casserole dish, spreading it evenly and pressing down gently.

Sprinkle shredded cheese over the top of the casserole, covering it entirely.

Bake the casserole for 25 to 30 minutes, until the cheese is bubbly and golden.

Let the casserole cool for 5 minutes, then garnish with parsley and crushed red pepper before serving.