To make the meatballs, squeeze the Italian sausage out of the casing and into a large bowl. Add the breadcrumbs, egg, and diced onion. Using your hands, mix the ingredients until combined. Form the mixture into about 24 ping pong ball sized balls.
Heat a large skillet over medium flame. Add the olive oil and tilt the skillet to coat. Add the meatballs and cook until browned on the outside (a couple minutes on each side). Work in two batches if needed to give the meatballs room to movel around. Once the meatballs are browned, remove them from the skillet to a clean plate.
Add the second half of the diced onion to the skillet, along with two cloves of minced garlic. Sauté in the leftover oil and fat from the meatballs until soft and transparent. Once soft, add the crushed tomatoes, oregano, basil, salt, pepper, brown sugar, and tomato paste. Stir until the tomato paste mixes into the crushed tomatoes.
Add the meatballs back to the skillet and gently stir to coat them in sauce. Place a lid on the skillet and let the meatballs simmer in the sauce for 15 minutes. Cook the pasta while the meatballs simmer.
Drain the pasta and serve each bowl with pasta, four meatballs, and a scoop of sauce.
To use store bought marinara, keep the browned meatballs in the skillet and pour a large jar of sauce over top. Allow the sauce to come to a simmer, then let the meatballs simmer in the sauce (with a lid) for 15 minutes.