Grate the Gruyère

• Grate the Gruyère and set it aside.

Make the béchamel

• Melt the butter in a saucepan, then stir in the flour.
• Add the milk a little at a time, stirring until the sauce thickens.
• Season with white pepper, salt and grated nutmeg, then stir in some Gruyère and a touch of Dijon mustard.

Slice the ham

• Cut the ham into slices.

Build the sandwiches

• Spread a layer of béchamel on each slice of bread.
• Add ham, Dijon mustard and grated Gruyère, then close the sandwich.
• Butter the outside of the bread on both sides.

Toast the sandwiches

• Cook the sandwiches in a hot pan until golden brown, flipping a few times.
• Let the cheese melt and everything get hot and gooey.

Finish and gratin

• Spread more béchamel on top and cover with more Gruyère.
• Melt the cheese with a kitchen torch, or bake the sandwiches in the oven.

Serve

• Cut each sandwich in half and serve immediately.