In a medium saucepan, melt the butter. Add the jalapeño and onion and cook over moderate heat, stirring, until softened, about 5 minutes. Stir in the rice and toast for 1 minute. Add the water and a generous pinch of salt and bring to a boil. Cover and cook over low heat until the grains are tender and the liquid is absorbed, about 18 minutes. Fluff the rice with a fork and stir in the cilantro. Let the rice sit, covered, for 5 minutes. Serve right away.
The rice can be refrigerated overnight and rewarmed.