Melt the butter in a medium saucepan. Add the onion and cook over moderately high heat until softened, about 5 minutes. Stir in the rice. Add the water and a pinch of salt and bring to a boil. Cover and cook over low heat for 18 minutes, until the rice is tender and the water is absorbed. Fluff the rice with a fork, cover and let stand for 5 minutes before serving.