For béchamel, melt butter in a saucepan over medium heat, add flour and whisk continuously until sand-coloured (2-3 minutes), then gradually add milk, whisking continuously. Season to taste, add nutmeg and whisk occasionally until thick and smooth (12-15 minutes). Set aside to cool slightly (5 minutes), then gently whisk in yolk and mustard, cover with plastic wrap, set aside to cool until needed. (This can be made a day ahead.)
Preheat oven to 180C.
Spread 1 tablespoons of béchamel onto each slice of bread. Divide ham and half the gruyere among half the bread slices, then sandwich with remaining bread slices with the béchamel side up. Scatter with remaining cheese.
12 medium slices, Wild Wheat white sourdough bread
300 grams Freedom Farms sliced manuka ham
200 grams gruyere cheese, grated
for the béchamel:
45 grams Butter coarsely chopped
45 grams Plain Flour
500 millilitres Lewis Road milk
Pinch of nutmeg
1 tablespoon Dijon mustard
salt and white pepper