Melt the butter in a big saucepan, then
stir in the flour and cook for 2 mins.
Gradually stir in the milk to a smooth,
lump-free sauce. Cook gently, stirring
constantly, until the sauce is nicely
thickened – about 5 mins. Stir in the
mustard, honey, vinegar and some
seasoning. Pour evenly over the potatoes.
Mix the crumbs and cheese together,
then scatter over the top and set aside
until ready to bake. You can cover the dish
and chill for up to 24 hrs before finishing.