Preheat the grill to 180°C/350F/gas mark 4. Lightly toast one side of each slice of bread then spread the untoasted side generously with mustard. Top 4 slices with a slice of ham and close to make a sandwich, place on a baking tray.
Bring one large pot of salted water to the boil. To make the cheese sauce add a large knob of butter to a sauce pan under a medium/low heat. Once the butter has melted add the flour and mix to a thick paste. Cook down for 1 minute then gradually add the milk, stirring with a whisk constantly so there are no lumps. Add the nutmeg and season with salt and pepper. Turn off the heat and add the cheese, stir until melted and the sauce it thick and smooth.
Top each sandwich with white sauce then place under the grill until golden brown. Meanwhile to make the poached eggs, turn the boiling water down to the very lowest heat possible, you don't want the water to be simmering at all, if it does on the lowest heat turn it off. Crack the eggs one at a time into ramekins and slowly pour the eggs into the water. Poach for about 2-5 minutes depending on how runny you want your egg. Remove with a slotted spoon and dry off on kitchen paper before topping each sandwich with a poached egg, season with salt and pepper.