Toss sliced cucumbers and ½ cup onion together in a large bowl.
Combine 1 cup cottage cheese, 3 tablespoons vinegar, 2 tablespoons chives, 2 tablespoons chopped dill, ⅛ teaspoon salt and ¼ teaspoon pepper in a blender; process until smooth and thick, about 1 minute. Transfer to the bowl with cucumbers and onions, stir until well coated. Garnish with dill fronds, if desired.
