Preheat your oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat. Brown the ground beef until fully cooked.
Add onions and cook until softened. Stir in garlic until fragrant.
Season the beef mixture with Italian seasoning, salt, pepper, and red pepper flakes.
Add marinara sauce and diced tomatoes. Simmer for 5 minutes.
Cook tortellini according to package instructions until al dente, then drain.
Combine tortellini with the meat sauce mixture.
Transfer everything to a baking dish and top with mozzarella and Parmesan cheese.
Bake for 20–25 minutes until bubbling and golden.
Let rest for 5 minutes before serving.