Preheat oven to 400°F. Adjust the oven rack to the middle position and butter a casserole pan. Set aside.
Meanwhile, combine ground beef, keto breadcrumbs, parmesan, parsley, egg, garlic powder, onion powder, salt and black pepper in a large salad bowl. Then, add the sauteed onions and mix everything well to combine.
Use a small cookie scoop to form the meatballs all the same size. Place the meatballs in the prepared casserole dish.
Bake them for 15 to 20 minutes or until fully cooked. Remove the meatballs casserole from the oven and drain the grease.
Top with marinara sauce and shredded cheese.
Place the meatballs casserole back in the oven. Bake for an additional 5 minutes until the cheese has fully melted.