Prepare the leek, cut into fine rings, wash well and drain. Melt 25 g butter in a wide pan, add the leeks and cook for 2-3 minutes. Sprinkle the flour on top, mix well, add the cream and let everything simmer gently for 2-3 minutes. Take off the fire.
Stir in the egg yolks and 100 g cheese, season with salt, pepper and nutmeg.
Toast the bread slices in 25 g butter on one side in a coated frying pan. Place with the roasted side up on an oven tray lined with baking paper, drizzle with a very little white wine. Put the leek mixture on top and sprinkle with the rest of the cheese. Bake in the oven at 200 degrees for 10-15 minutes.