Preheat the oven to 400 degrees F. and grab an 8x8-inch baking dish.
To a large mixing bowl, add the 1 pound ground beef, ½ cup shredded mozzarella cheese , ¼ cup grated parmesan cheese, 1 large egg, ½ cup chopped onion, 1 teaspoon garlic powder, ½ teaspoon Italian seasoning, ¼ teaspoon salt, and ½ teaspoon ground black pepper.
Using a large wooden spoon or clean/gloved hands, mix all of the ingredients together until they stick together to form a large ball.
Using a medium scoop or clean/gloved 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, make sure they are not touching each other. They will shrink in size while baking.
Place the meatballs into the oven to bake for about 15 to 20 minutes depending on the size of your meatballs.
Carefully 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 them in a separate bowl. Completely drain the dish of any fat and evenly coat the bottom of the dish with 12 ounces of pasta sauce.
Place the meatballs evenly on top of the sauce. Top the meatballs with the rest of the leftover 12 ounces of pasta sauce. Make sure each individual meatball is covered in the pasta sauce. Use a spoon to spread it evenly.
Top the meatball casserole with 1 cup shredded mozzarella cheese spread out in an even layer. Place into the oven and bake for an additional 10 to 15 minutes, until the cheese is melted and bubbly. Carefully remove the casserole from the oven. Serve warm on top of your favorite pasta or with a side of your favorite vegetable. Enjoy!
Store any leftovers in an airtight container in the refrigerator for up to 3 days.