Peel potatoes, dice finely and boil in a pot, barely covered with salted water, for about 15 minutes until soft. Drain the water and let the potatoes steam out briefly with the lid halfway on. While potatoes are cooking, heat milk and butter on lowest setting in a small saucepan. Quarter, core and finely slice the cucumber.
Add milk-butter liquid with mustard and chives to potatoes, mash thoroughly with a potato masher, season with salt, pepper and nutmeg. Finally, mix in the sliced cucumber with a spoon and season again.
Pour the cucumber capü into a suitable tumbling cup, arrange on a plate, garnish and serve.