Spread each slice of bread with mustard. Make sandwiches by putting each slice of cheese against the mustardy bread, and a slice of ham between them. Cut each sandwich in half to make two triangles.
Squish the sandwich triangles into an ovenproof dish approx. 27 x 21cm x 6cm deep.
Beat together the eggs, salt and milk (I measure out the milk into whatever the mustard’s been in for maximum flavour penetration) and then pour this over the sandwiches tightly packed in the dish.
Cover the dish with clingfilm and leave in the fridge overnight (I soaked mine for closer to 2 hours and it was fine).
Next morning, preheat the oven to 200°C (180°C fan forced). Take the dish out of the fridge and remove the clingfilm.
Sprinkle with the grated cheese and Worcestershire sauce and bake in the oven for 25 minutes.