Peel and dice the onions. Peel, wash and cut the cucumber if necessary.
Steam the onions in the hot oil until translucent. Briefly braise the cucumber pieces. Take the pan off the stove and puree the vegetables. Season with vegetable stock and stir in the buttermilk. Season to taste with salt, pepper and cayenne pepper. Thicken with cornstarch if necessary.
Possibly heat up again slightly. Do not heat too much, otherwise the buttermilk will flake.