Rinse rice in a sieve until water runs clear and drain well. Cook cumin and mustard seeds in oil in a 2-quart heavy saucepan over moderate heat, stirring, until mustard seeds begin to pop, about 2 minutes. Stir in garlic and rice and cook, stirring, 1 minute. Add water and bring to a boil, uncovered. Cover rice and cook over very low heat until water is absorbed and rice is tender, about 20 minutes. Remove pan from heat and let rice stand, covered, 5 minutes. Fluff rice with a fork and stir in scallions and salt and pepper to taste.
