Transfer the tomatoes and liquid to a bowl and gently crush the tomatoes with your hands.
Heat the oil in a saucepan, set over moderate heat until hot. Add the onion and garlic and cook, stirring occasionally, for 5 minutes. Add the tomatoes and liquid, tomato sauce, basil, salt, and red pepper flakes, bring to a boil and simmer for 15 minutes.
Meanwhile, cook the pasta. Drain the pasta and transfer to a large bowl. Add sauce and toss to coat. Top with cheese. Serve with a green salad and crusty bread.
Variations: Mushroom Marinara Sauce: After onion and garlic have softened, add 1/2 pound fresh sliced mushrooms and cook, stirring occasionally, for 5 minutes. Continue with recipe as above.
Tuna Marinara Sauce: Add 1 can Italian tuna, packed in oil, including oil, after sauce has simmered 10 minutes, and cook for an additional 5 minutes.
Sausage Marinara Sauce: After onion and garlic have been softened, add 1/2 pound crumbled Italian (sweet or hot) sausage and cook, stirring, until no longer pink. Add tomatoes and continue with recipe as above.