Remove skin from cucumbers, chop coarsely. Also chop both peppers, celery, green onions, peeled potatoes and garlic. Put the vegetables in a blender, pour in the grape juice and grind so that you get a not too homogeneous puree. Beat the egg, add to the blender along with salt and pepper; beat it all together again. Refrigerate the soup for 2-3 hours. Serve with wholemeal buns.