Peel the cucumber. Grate it using the large holes on a kitchen grater. Pour away the liquid.
Mix the chopped dill and mint into the yogurt. Peel and mince the garlic clove, then add it to the yogurt along with the salt. Fold in the drained cucumber and stir vigorously for 1-2 minutes. Taste and adjust seasoning if needed.
Refrigerate until chilled, then serve as an accompaniment to main courses or with flatbread as a starter.