Preheat the oven to 400 degrees F. Line a large baking sheet with foil and lightly spray the foil with olive oil cooking spray.: You will notice the oven warming the kitchen as the heat builds, which helps the meatballs cook evenly. A foil lined baking sheet makes cleanup simple and reflects heat for even browning. Lightly spraying prevents sticking and encourages a crisp exterior. If your oven runs hot, place the rack in the center to avoid overly rapid browning. A common mishap here is skipping the spray, which can make meatballs adhere to the pan and break apart when flipped or removed.
Add all meatball ingredients (except marinara sauce) to a large bowl and use your hands to combine. Scoop the meat mixture into 3-tablespoon-sized balls and arrange them on the prepared baking sheet. You should get about 24 meatballs.: As you mix, feel for an even distribution of ingredients, with the grated onion and parsley folded in gently. The mixture should be cohesive but not overworked, because over mixing compresses the proteins and yields dense meatballs. When rolling, use slightly damp hands to prevent sticking, and aim for uniform size so they bake consistently. A frequent mistake is squeezing the mixture too tightly; let the meatball hold its shape without compacting it.
Bake the meatballs until they are cooked through (ground beef should be cooked to an internal temperature of 160 degrees F, which you can check with an instant-read thermometer), about 15 to 20 minutes. If you want to brown the tops, broil the meatballs briefly after baking. Add the meatballs to a large bowl and toss with 1 jar of warmed marinara sauce.: During baking you will hear faint sizzling as the exterior sets and the aroma of cooking beef and onion fills the air. Look for an even golden brown color on the outside and juices running clear near the bottom. The internal temperature is the best indicator of doneness, ensuring safety without overcooking. Broiling briefly adds attractive caramelized spots, but watch closely because sugars in the sauce or cheese can burn quickly. A pitfall here is removing meatballs too early because of color alone; always verify temperature for safety.
Toss the cooked spaghetti with 1/2 jar of warmed marinara sauce. Place the meatballs on top of the spaghetti, and serve along with the remaining 1/2 jar of marinara sauce to spoon on top, if desired.: The sound of sauce slipping around noodles is comforting, and the sauce should cling, not pool, when the pasta is properly sauced. Toss gently so the spaghetti is evenly coated and the strands remain distinct. Placing meatballs on top showcases them visually, and reserving sauce allows guests to add more for a saucier plate. Avoid drowning the pasta in sauce early, which can make the noodles soggy if held before serving.
Serve with fresh basil leaves and freshly grated parmesan cheese for topping.: The final garnishes add aroma and a bright finish, with basil contributing a fresh herbal scent and Parmesan melting slightly into warm sauce. Scatter leaves just before serving so they retain their color and fragrance. Grating cheese fresh creates delicate, airy shreds that melt beautifully; pre grated cheese can feel chalky. A common oversight is adding garnishes too early, which diminishes their visual and aromatic impact.