I am doing more adapting now: having added the tomatoes I will continue to cook the sauce on low, uncovered, for 20 mins. This boils off excess liquid to bring out more flavor. The recipe calls for chopped fresh plum tomatoes and a cooking time on high heat for 6 to 8 mins with frequent stirring. Add salt and generous grindings of pepper to "balance the sweetness of the onion" as Marcella says. If using oil, the signal that the sauce is done is when the oil floats free of the tomatoes. Boil pasta al dente, toss together with the sauce and serve with grated parmiggiano cheese.