Peel and cut the potatoes into pieces.

Boil in salted water until tender.

Drain and transfer to a bowl.

Warm the milk and mash it into the potatoes.

Cover the bowl to keep the potatoes warm.

Crush the mustard seeds and peppercorns with a mortar and pestle or in a spice grinder.

Peel and dice the onion.

Heat the butter in a skillet and saute the onion until transparent.

Add the mustard seeds and pepper corns and saute for a minute.

Add the apple sauce and sugar and cook, stirring for another few minutes.

Stir in the mustard.

Fold the apple sauce mixture into the potatoes.

Season with salt, pepper, and sugar to taste and serve.