To a large mixing bowl, add in the ground beef, ½ cup mozzarella, parmesan, egg, chopped onion, garlic powder, Italian seasoning, salt, and ground black pepper.
Stir together, using a large spoon on by hand, until all of the ingredients are combined and the meat mixture sticks together to form a large ball.
Using a scoop or by hand, form the meatballs any size that you like. Place the meatballs in a casserole dish leaving a small amount of space between each meatball. Basically just make sure they are not touching each other or the side of the dish.
Place into the preheated oven and bake at 400 degrees F. for 15 to 20 minutes depending on the size of your meatballs.
Remove the meatballs from the oven and drain off the excess fat. It may be easier to use a slotted spoon and remove the meatballs and place in a small bowl or dish for a moment. Completely drain the dish of any fat and evenly coat the bottom of the dish with half of the pasta sauce.
Place the meatballs evenly on top of the sauce. Top the meatballs with the rest of the pasta sauce. Make sure each individual meatball is covered in the pasta sauce. Use a spoon to spread it evenly.
Top the keto meatball casserole with the shredded mozzarella cheese spread out in an even layer. Place into the oven and bake an additional 10 to 15 minutes, until the cheese is melted and bubbly.
Serve warm on top of your favorite pasta or with a side of your favorite vegetable. Store any leftovers in an airtight container in the refrigerator up to 3 days.