Peel and dice the potatoes and cook in a saucepan, just covered with salted water, for approx. 15 minutes until soft. Pour off the water and let the potatoes steam briefly with the lid half on.
While the potatoes are boiling, heat the milk and butter on low heat in a small saucepan.
Quarter, core and finely slice the cucumber. Add milk-butter liquid with mustard and chives to the potatoes, thoroughly mash everything with a potato masher, season with salt, pepper and nutmeg. Finally mix in the sliced cucumber with a spoon and season again to taste. Fill the cucumber cap into a suitable tumbler, arrange on a plate, garnish and serve.