This dish is a lovely afternoon snack. Add as much Worcestershire sauce as you can — it cuts brilliantly across the sweet leeks.
This recipe was adapted from Eat Your Vegetables by Arthurs Potts Dawson (Octopus/Mitchell Beazley 2012).
Melt the butter in a saucepan until it foams. Mix in the flour and cook over low heat, stirring for 4-5 minutes without allowing the roux to brown. With the pan still on the heat, slowly whisk in ½ the milk, then increase the heat, bringing to a boil, and cook until thickened, whisking well to avoid lumps. Season lightly with salt but quite generously with pepper and nutmeg, then add the Cheddar and Parmesan. Keep the sauce warm until you're ready to make your toast.
Using a very sharp knife, cut the whites of the leeks lengthwise into quarters. Wash them under cold running water to remove any sand or soil, then finely chop.
Melt the butter in a saucepan, add the leeks and a pinch of salt, and cook over medium heat for 10 minutes, until they are soft and transluscent, stirring occasionally. Season with salt to taste and add both sauces and mix well. Preheat the broiler to medium.
When the leek mixture is hot, toast the bread lightly on one side under the broiler, then turn the slices over and spread the leek mixture onto the untoasted side. Sprinkle the Cheddar on top and put the toast back under the broiler to color the cheese to a golden brown. Serve hot with your favorite pickle.