FOR SAUCE: Crush tomatoes in their juice by hand in a big bowl - do not drain. Heat oil in heavy large pot over medium-low heat. Add onion, sauté until golden, about 10 minutes. Add garlic; stir 1 minute. Add tomatoes with juices and 2 tablespoons basil; bring to a boil. Reduce heat; simmer until sauce thickens,about 1 hour. Mix in 2 tablespoons basil. Season with salt and pepper. Set sauce aside.

FOR MEATBALLS: Preheat oven to 350ºF. Lightly oil baking sheet. Mix crumbs and milk in medium bowl; let stand 5 minutes. Mix in Parmesan, onion, basil, egg, garlic and pepper. Add sausage and blend well. Using wet hands, form mixture into 1 1/4-inch balls. Place on baking sheet. Bake until meatballs are light brown and cooked through, about 30 minutes. Add to sauce.

Have the sauce at a low simmer by the time the spaghetti is done and place a ladle-full (or two) of sauce in a big pasta bowl. Cook spaghetti in large pot of boiling salted water until al dente. Drain (do not rinse) or fish out the spaghetti with long tongs or a pasta net and place pasta in the bowl with the ladle of sauce. Toss the hot spaghetti with the small amount of ladled-out sauce. Spoon additional sauce and meatballs over spaghetti to serve and garnish with fresh basil leaves.