Preheat the oven to 374 degrees. Grease 4 x 1 cup ramekins with butter. Tip in the breadcrumbs and shake from side to side to coat. Tip out and discard the excess breadcrumbs. Chill until set. Pop a baking sheet in the oven.
Melt the butter in a large pan. Stir in the flour and cook for 1 minute. Reduce the heat and gradually blend in the milk, stirring, until thickened. Remove from the heat, beat in the yolks, cheeses and mustard. Season.
Whisk the egg whites to stiff peaks. Use a metal spoon to mix a quarter into the mixture, then fold in the remainder. Fill each ramekin and smooth the top. Put on the baking sheet and cook for 20 minutes (don’t open the oven!) until risen and golden brown. Serve immediately.