Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, cook ground beef and diced onion until beef is fully browned and onion is translucent.
Drain excess grease from the skillet and return to heat.
Add minced garlic, oregano, salt, and black pepper to the beef mixture and sauté for 60 seconds.
Stir in the marinara sauce and reduce heat to low, simmering for 5 minutes.
Boil tortellini in a large pot of salted water for 2 minutes less than the package instructions; drain immediately.
Fold the partially cooked tortellini into the beef and sauce mixture until evenly coated.
Transfer the mixture into the prepared baking dish and spread evenly.
Sprinkle shredded mozzarella and grated parmesan cheese over the top.
Bake for 20 minutes or until the cheese is melted and starting to brown slightly.
Remove from oven and garnish with fresh parsley before serving.