1. For the white sauce: Start by melting the butter in a pan. Add the flour and stir into the butter. Then gradually add the milk while beating. Let it cook for a couple of minutes to minimise the floury taste. Season to taste with salt, pepper and nutmeg. Remove from the heat.
2. Stir the crème fraiche and the mustard together, and spread the mixture onto the bread slices. Place two slices of ham on two bread slices, followed by slices of Marquis cheese. Close the sandwich.
3. Fry the sandwich in the butter, about 2-3 minutes on each side. The sandwiches must not brown, so fry over a medium heat.
4. Remove the sandwiches from the pan. Drizzle with white sauce before topping with grated Aged Havarti. Place under the grill in the oven until the cheese has melted.
5. In the meantime, fry two eggs – one for each sandwich. Place on top. Serve immediately.