To make the mornay sauce, melt the butter then add the flour and beat through until smooth.
Start incorporating the milk in stages, constantly stirring so it doesn’t go lumpy.
Season to taste with salt, pepper, and nutmeg before adding 80g cheese in stages.
Turn off the heat and beat through the egg yolk.
Spread Dijon mustard on the inside of the bread, top with ham, gruyere, and some of the mornay sauce.
Melt butter in a pan then add the open sandwiches bread side down when golden and crispy, fold together.
Top with more sauce and cheese, place under a hot grill until golden and bubbling.
Melt butter with a splash of oil to fry the egg and season with salt/pepper.
Top the sandwich with the eggs and optional chives.
