Cook over medium-high heat, stirring, until the berries soften and release their juice. Stir to a smooth paste:
1 tablespoon water
1 1/2 teaspoons cornstarch
Briskly stir the cornstarch mixture into the berries and cook until thickened, about 1 minute. If using the sauce with a warm or room-temperature food, stir in:
1 to 2 tablespoons unsalted butter (optional)
Serve at once, or let cool then cover and refrigerate for up to 3 days. Reheat over low heat.