Preheat the oven to 375 degrees F.
Warm a large, oven-safe skillet over medium-high heat. Add the ground beef and onion. Cook for 3-4 minutes, until the onion is softened and the beef mostly browned. Drain any excess fat.
1 pound lean ground beef, 1/2 medium yellow onion
Add garlic, Italian seasoning, 1/2 teaspoon salt, and 6-8 cracks of pepper to the pan. Cook for 1-2 minutes more, stirring well.
2-3 cloves garlic, 2 teaspoons Italian seasoning, kosher salt and fresh-cracked black pepper
Stir in the marinara sauce, uncooked tortellini, and a little more than half of the mozzarella. Top with remaining mozzarella and the Parmesan.
24 ounces marinara sauce, 18-20 ounces refrigerated tortellini, 1 and 1/2 cups shredded mozzarella cheese, 2 Tablespoons grated Parmesan cheese
Cover tightly with foil, or a lid if your pan has a tight-fitting one. Bake, covered, for 30 minutes.
Remove the foil or lid. The cheese should be melted, the tortellini tender, the sauce bubbling. (Test a tortellini from the middle, not from the top, to be sure they're all fully tender.)
To lightly brown the top, if desired, turn the oven to broil for 1-2 minutes, watching carefully to be sure the top does not burn. Remove from the oven, let rest for 3-4 minutes, then sprinkle lightly with chopped parsley or basil. Serve and enjoy!