Heat the olive oil in a large non-reactive pot over moderate heat. Add the onion and saute until translucent, about 8 minutes. Add the parsley and garlic and cook briefly to release their fragrance. Add the tomatoes, basil and salt. Simmer briskly until reduced to a sauce like consistency, stirring occasionally so nothing sticks to the bottom of the pot. The timing will depend on the ripeness and meatiness of your tomatoes and the size of your pot. If the sauce thickens too much before the flavors have developed, add a little water and continue cooking.
Taste and adjust the seasoning. If the sauce tastes too acidic, add the baking soda and cook for 5 more minutes. If it needs a touch of sweetness, add the sugar and cook for 5 more minutes. Remove the basil stem before serving.