In a 1 1/2 quart heavy saucepan cook shallot in oil over moderately low heat, stirring occasionally, until softened. Add mustard seeds and cook, stirring occasionally, 1 minute. Stir in rice, salt, and water and bring to a boil. Stir rice once and cook, covered, over very low heat 15 minutes. Remove pan from heat and let pilaf stand, covered, 5 minutes. Fluff pilaf with a fork and stir in scallions.