Step 1

Preheat the oven to 375 degrees Fahrenheit.

Step 2

Bring a large pot of salted water to a boil. Cook the spaghetti to the package directions for al dente, about 8 to 12 minutes. Drain the spaghetti and toss it with half of the marinara sauce. Set aside.

Step 3

While the pasta is cooking, heat the olive oil in a large skillet over medium-high heat. Add the onion and cook until softened, about 5 minutes.

Step 4

Add the garlic, salt, paprika, black pepper, and crushed red pepper flakes to the pan. Cook until the garlic is fragrant, about 1 minute. Remove the pan from the heat and let the mixture cool slightly.

Step 5

In large bowl, combine the ground beef, Italian sausage, egg, Parmesan, breadcrumbs, parsley, and the cooled onion mixture. It's best to use your hands to combine the meatball mixture, but a large spoon would work here, too. When the ingredients are combined, add the water and squish it into the meat mixture using your hands.

Step 6

Form the mixture into 12 meatballs slightly larger than a golf ball, rolling the meatballs until they're smooth and round.

Step 7

Wipe out the onion skillet or clean it with soap and water if it has any burnt bits on the bottom. Add about 1/4 inch of oil to the pan. Heat the oil over medium heat and add the meatballs, cooking in batches if necessary to keep from crowding the pan. Cook the meatballs until they're browned on all sides and almost cooked through, about 10 minutes.

Step 8

Spread a spoonful of the remaining marinara sauce to a 9x9 baking dish. Add the marinara-tossed spaghetti and top it with the meatballs.

Step 9

Top the meatballs with the remaining marinara and sprinkle the top with the mozzarella.

Step 10

Cover the baking dish with aluminum foil and bake it until the cheese is melted and the meatballs register 165 degrees on an instant-read thermometer, about 15 to 20 minutes. Remove the foil for the last 5 minutes of cook time to brown the top of the mozzarella.