Step 2

Warm the milk with half the butter in a small pan until the butter has melted. Do not let it come to the boil.

Step 3

Beat the warm milk mixture into the potatoes a little at a time, then fold through the wholegrain mustard. Season generously with salt and pepper.

Step 4

Melt the rest of the butter in a skillet over low heat and soften the scallions for about 2 minutes. They should stay bright green rather than browning.

Step 5

Stir the buttery onions through the champ and take it to the table straight away, while it is still hot enough to melt a pat of butter on top.