Step 1

Preheat the oven to 375°F. Lightly grease a 9x13 baking dish.

Step 2

In a large skillet, cook the ground beef and onion over medium-high heat until beef is browned, about 5-7 minutes. Drain excess fat. Add garlic and cook for 1 minute more.

Step 3

Stir in the marinara sauce, diced tomatoes, oregano, basil, and red pepper flakes. Bring to a simmer and cook for 5 minutes. Season with salt and pepper to taste.

Step 4

Add the fresh spinach to the sauce and stir until wilted. Remove from heat.

Step 5

Spread half of the sauce in the prepared baking dish. Top with the uncooked tortellini in an even layer. Dollop the ricotta evenly over the tortellini. Pour the remaining sauce over the top.

Step 6

Sprinkle the mozzarella and Parmesan cheeses evenly over the casserole. Cover tightly with foil.

Step 7

Bake for 30 minutes. Remove foil and bake for 5-10 minutes more, until the cheese is melted and bubbly and the tortellini is tender.

Step 8

Let stand for 5 minutes before serving. Garnish with fresh basil or parsley if desired. Enjoy!