Peel and chop the onion and fry gently in duck fat (or oil) until the onion is limp and translucent. Meanwhile, grate (shred) the cheese and separate the egg whites from the yolks. Cut the crusts from the bread and crumble the soft white interior between finger and thumb to make fine breadcrumbs (if you have a food processor, you can use the chopper attachment to do this for you). Chop the bacon into small pieces and add to the onion in the pan for a couple of minutes before taking the pan off the heat.
Place the onion, bacon, cheese, breadcrumbs, milk and egg yolks into a large bowl and mix well together. Season with a little salt and freshly ground pepper. Beat the egg whites until they are stiff then fold them gently into the mixture (do not beat again).
Pour this mixture into a well-buttered baking tin about 8" x 5" (20cm x 12.5cm) and bake for 20 minutes in a preheated hot oven (Mk 7 - 425ºF - 220ºC). Serve hot with Fresh Tomato Salad