01 -
Heat the oven to 200°C / 400°F. Put potatoes in a large saucepan, cover with cold water and bring to a boil. Simmer 10 minutes, then add cauliflower and cook another 8–10 minutes until both are very tender.
02 -
Drain thoroughly and leave in the colander for three minutes so excess steam escapes. Return vegetables to the dry warm pan, off the heat.
03 -
Mash by hand with butter. Gradually stir in warm milk, mustard, salt and pepper. Keep a little texture from the cauliflower rather than blending into a gluey puree.
04 -
Fold in 80 g cheddar and the chives. Spoon into a baking dish about 20 × 25 cm, level loosely and make shallow ridges across the surface with a fork.
05 -
Scatter the remaining 40 g cheddar over the top. Bake 18–20 minutes until hot through and golden at the edges. Let stand 10 minutes before scooping into six side portions.
06 -
Serve with a main dish of your choice prepared separately. Refrigerate leftovers within two hours for up to three days, then reheat covered until steaming hot throughout.