Cook onion, celery, carrot, and fennel in butter in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until onion is softened, 5 to 6 minutes. Add quinoa and sauté over moderately high heat, stirring, until lightly toasted, 2 to 3 minutes. Add water and salt and pepper to taste and cook over moderately low heat, covered, until quinoa is tender and liquid is absorbed, 12 to 15 minutes.
