Preheat and prep. Heat the oven to 375°F (190°C).
Lightly coat a 9x13-inch baking dish with nonstick spray.
Brown the beef. Warm a large skillet over medium-high heat. Add the ground beef and break it up with a spatula. Cook until no pink remains, 5–7 minutes.
If the pan is dry, add a splash of olive oil.
Sauté aromatics. Add the diced onion and a pinch of salt. Cook until softened and lightly golden, 4–5 minutes. Stir in the garlic and cook 30 seconds, until fragrant.
Season the sauce. Sprinkle in Italian seasoning, black pepper, and red pepper flakes (if using).
Pour in the marinara and beef broth. Stir and bring to a gentle simmer for 3–4 minutes to meld flavors. Taste and adjust salt.
Add the creaminess. Reduce heat to low.
Stir in the Greek yogurt and ricotta until the sauce turns silky and pinkish. Keep it below a simmer so the dairy doesn’t split.
Fold in the tortellini. Add the refrigerated tortellini directly to the sauce. Stir to coat evenly.
The pasta will finish cooking in the oven.
Assemble the casserole. Transfer the mixture to the prepared baking dish. Top evenly with mozzarella and Parmesan.
Bake. Cover loosely with foil (tent so cheese doesn’t stick). Bake for 15 minutes, then remove foil and bake another 10–12 minutes, until the cheese is melted and bubbling and the edges are lightly browned.
Rest and garnish. Let the casserole rest 5–10 minutes to set.
Sprinkle with chopped parsley or basil before serving.
Serve. Scoop generous portions and pair with a crisp salad or steamed veggies for balance.