Cook the Meat. In a skillet over medium heat, add olive oil and sauté the diced onion and garlic until soft, about 2-3 minutes. Add the ground beef or sausage and cook until browned, breaking it up with a spoon. Drain excess fat.
Prepare the Sauce. Stir in the marinara sauce, Italian seasoning, and red pepper flakes (if using). Simmer for 5-10 minutes to let the flavors meld.
Cook the Tortellini. While the sauce simmers, cook the tortellini in a pot of boiling water according to package instructions. Drain well.
Assemble the Dish. In a large baking dish, spread a thin layer of the meat sauce on the bottom. Layer half of the cooked tortellini, followed by dollops of ricotta cheese. Add more sauce and the remaining tortellini. Top with the rest of the sauce, then sprinkle with mozzarella and Parmesan cheese.
Bake. Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Garnish and Serve. Remove from the oven and let it sit for a few minutes before serving. Garnish with fresh basil, if desired. Enjoy your cheesy Baked Tortellini with Tomato and Meat Sauce!