sour cherries
Small, dark red cherries with a distinctly tart, aromatic flavour, usually cooked or baked rather than eaten raw.
Fresh sour cherries appear only briefly in summer, mostly at farmers' markets. Jarred morello cherries, canned or frozen tart cherries are easier to find in larger supermarkets, Eastern European shops or online.
Substitute:
Sweet cherries with a little lemon juice come close, though they taste softer and less aromatic. With jarred cherries, drain off the syrup and use less sugar where needed.