Combine ground beef and onion, season to taste with salt and pepper, and roll the meat into golfball-sized balls. Season a frying pan with a little oil and brown the meatballs on all sides.
Mix the cooked spaghetti with 3.5 oz parmesan (saving a little for garnishing) and 1 tbsp olive oil. When well-coated, swirl onto a fork against a spoon until you have a portion sizeable enough and gently press into the muffin form.
Add a spoonful of tomato sauce to the middle of the noodle nest, place a meatball on top, follow with another dollop of sauce and sprinkle with parmesan. Bake at 375 °F for 20 minutes. When ready to serve, garnish with more parmesan and chopped basil leaves.