Short-grain rice adds the perfect texture to this traditional dish from northern Portugal. The salt cod may eliminate the need for additional salt. BROWN THE ONION in the olive oil in a large heavy saucepan over moderate heat. Meanwhile, shred the cod, remove the skin and bones and wash but do not soak. Add the cod to the onion, cover and simmer for about 10 minutes. Add the tomatoes and leave to cook 20-to-30 minutes, covered, over low heat. Meanwhile, wash and drain the rice. Add 3 1/3 cups water to the pan (add twice as much water as there is rice), season to taste with salt and pepper. When the water boils, add the rice and continue to cook on low heat until the rice is done--probably 25-to-30 minutes. Cover the kettle for the last half of cooking.