Combine blueberries, raspberries, sugar and water In a large saucepan,. Bring to a simmer, stirring occasionally.
Mix lemon juice and cornstarch; add to the berry mixture and cook, stirring, until slightly thickened, about 1 minute. Stir in creme de cassis (or black currant syrup), if desired. Let cool slightly; serve warm.
Make Ahead Tip: Cover and refrigerate for up to 4 days.