Heat oil in a 3-quart heavy saucepan over moderate heat. Add shallots, mustard seeds, and pepper flakes; cook, stirring occasionally, until shallots are softened, about 5 minutes. Stir in remaining ingredients and simmer, stirring occasionally, until berries just begin to pop, about 7 minutes. Let stand until cool, then transfer to a serving bowl, and cover and keep chilled until ready to serve. Makes 4 cups.