Wash, peel & chop tomatoes, discarding seeds. Heat oil in a moderately deep pan, add onion & garlic and saute until onion is limp. Discard garlic and add tomatoes, chopped basil and salt & pepper. Simmer for about 20 minutes over moderate heat. While sauce is simmering, bring a large pot of water to boil. Add salt to taste and add spaghetti. Boil until spaghetti is "al dente," aprox 8 to 10 minutes. Drain spaghetti, toss with the sauce and the grated cheese. Garnish each plate with some of the basil leaves. Serve at once.