Heat oven to 190C/170C fan/gas 5. Mix together 100g of the grated Gruyère, crème fraîche and Dijon mustard. Season with salt, black pepper and a pinch of nutmeg and spread half the mixture over 5 slices of white bread. Top with a layer of cooked sliced ham and another slice of bread.
Spread softened butter over one side of the sandwich and place on a baking sheet, butter-side down. Top the unbuttered side with the rest of the cheese mixture the remaining 50g grated Gruyère. Can be assembled the day before and chilled. To serve, bake in the oven for 20 mins or until golden, then slice the sandwiches into bite-sized pieces and serve straight away.