This is an old favourite. It is traditionally made with chicory (endives) rather than leeks, but for St David's Day, which it is, one wants something in which leeks are a central feature, rather than an "also-ran".
4 small or 2 large leeks, trimmed. If large, cut in half widthways.
4 slices ham - the nice kind, without added water
250 ml milk
1 heaped tsp plain flour
A very little salt (only a pinch - the ham will add a great deal)
Dry mustard powder
Knob of butter
c 75g grated cheese (cheddar, emmenthal, Parmesan, one of those bought mixes....)
Boil, or (preferably) steam the leeks until tender. When cool enough to handle, wrap each leek or 1/2 leek in a slice of ham. Place side-by-side in an oven-proof dish.
Meanwhile, whisk the flour, milk and seasoning together and melt the butter in a saucepan. Pour the milk mixture on to this and bring to the boil, stirring all the time. Stir in half the grated cheese, then pour this over the ham and leeks. Top with the rest of the grated cheese. Bake in a moderate oven for 30-45 minutes.
This is traditionally served with mashed potato.