Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat. Add onion and cook for 3-4 minutes until softened.
Add garlic and cook for 30 seconds until fragrant.
Add ground beef and cook until browned, breaking it apart as it cooks.
Stir in marinara sauce, Italian seasoning, salt, and black pepper. Simmer for 5 minutes.
Spread a thin layer of meat sauce in the bottom of a greased 9x13-inch baking dish.
Arrange half of the frozen dumplings in a single layer over the sauce.
Spread half of the ricotta cheese over the dumplings.
Sprinkle one-third of the mozzarella cheese on top.
Add another layer of meat sauce.
Repeat layers with remaining dumplings, ricotta, sauce, and mozzarella.
Finish with remaining mozzarella and Parmesan cheese.
Cover loosely with foil and bake for 30 minutes.
Remove foil and bake for an additional 15 minutes until bubbly and golden brown.
Allow the casserole to rest for 10 minutes before serving.
Garnish with fresh parsley and serve warm.