Melt the butter in a saucepan, then stir
in the flour and mustard powder and cook
for 1 min. Gradually stir in the milk until
smooth with no lumps, then keep stirring
until the mixture begins to bubble and
thickens to a creamy sauce. Remove from
the heat, then stir in all but a handful of the
Heat oven to 220C/fan 200C/gas 7 and
bring a large pan of water to the boil. Cook
the potato slices for 5 mins, tip in the
broccoli and cauliflower for another 3 mins,
then finally add the peas for 1 more min.
Drain all the veg and pat dry. Reserve enough
potato slices to cover the top of the finished
dish, then gently stir the rest of the
vegetables into the sauce with the chives.
Tip into a deep ovenproof dish, arrange
over the reserved potato slices, then sprinkle
with remaining cheddar. Bake for 20-25 mins
until the topping is golden and crisp, then
serve straight from the dish.