Peel cucumber, remove seeds, and slice. Reserve 4 slices. Put remainder in blender or food processor with broth, onion, mint, salt, garlic powder and lemon juice. Process until finely chopped but not liquified. Combine sour cream and yogurt in a bowl; stir in cucumber mixture. Mix well and cover tightly. Refrigerate several hours to blend flavours before serving. Spoon into chilled bowls. Garnish with reserved thing cucumber slices.