First melt the butter in a pan, stir in the fl our and cook for a minute or two before adding the milk bit by bit, stirring all the time. Cook the flour mix for 2 minutes, take the pan off the heat. Stir in the mustard, Worcestershire sauce, Tabasco, a little salt and freshly ground pepper.
Stir beaten egg yolks into the sauce and leave it on one side to cool for a minute.
Now toast the bread on both sides. Then fold the grated cheeses into the sauce mixture. Whisk the egg whites until stiff and, using a metal spoon, carefully fold them into the cheese mixture.
Cover one side of the toast liberally with the cheese mixture. Pop the slices under a medium grill and when the tops are golden brown, light and puffy, serve at once. A slice of fresh tomato complements the dish perfectly.