1. In a heavy 3- to 4-quart saucepan, heat the oil over moderate heat for 30 seconds, tipping the pan to coat the bottom evenly. Add the onion and cook, stirring constantly, for 5 minutes, or until it is soft and transparent but not brown.
2. Pour in the rice and stir for 2 to 3 minutes, until all the grains are coated with oil. Do not let the rice brown. Add the stock, water, tomatoes and salt, and return to a boil, still stirring.
3. Cover the pan and reduce the heat to its lowest point. Simmer for 20 minutes, or until the rice has absorbed all the liquid. If the rice must wait, drape the pan loosely with a towel and keep it warm in a preheated 250° oven.
