In a small bowl, dissolve the achiote paste in the water. In a medium saucepan, heat the chicken fat or oil. Add the onion and cook over low heat until softened, about 5 minutes. Add the rice and stir well. Add the achiote water and the salt and bring to a boil. Cover the saucepan and cook over low heat until the water has been absorbed and the rice is tender, about 20 minutes. Remove from the heat and let steam, covered, for 5 minutes. Fluff the rice and serve right away.
Achiote or annatto seed paste (recado rojo) is available at Latin markets and at specialty food shops.
Chicken with Mulato Chile Sauce