In a large skillet, cook the ground beef over medium-high heat until browned. Drain off any excess grease.
Add the chopped onion to the skillet and cook until softened, about 5 minutes. Stir in the minced garlic and cook for 1 minute more until fragrant.
Pour in the marinara sauce and water. Add the Italian seasoning, salt, and pepper. Stir to combine.
Bring the sauce to a simmer, then add the refrigerated tortellini. Stir gently to coat the tortellini with the sauce. Cook for 2-3 minutes, until the tortellini are slightly tender.
Pour the tortellini and sauce mixture into a greased 9x13 inch baking dish.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese evenly over the top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly and the edges are lightly browned.
Let stand for 5 minutes before serving.
Notes
Serve hot with a side salad and crusty bread for a complete meal.