Wash the leeks, and cut the green part, leaving only the whites. Make an incision lengthwise to wash them while keeping their shape. Steam for about 20 minutes.

Meanwhile, make the béchamel: In a small saucepan, melt the butter over low heat. Add the flour and stir to obtain a "roux" (brown mixture). Pour the cold milk, then the heavy cream and bring to avoid, stirring constantly until the sauce thickens. Add salt, pepper and nutmeg.

Preheat the oven at 400°F. Wrap each leek in a slice of smoked salmon.

Place the salmon wrapped leeks in a buttered baking dish. Pour the béchamel sauce over them.

Sprinkle with the Swiss cheese, and bake for about 15 minutes.

Wash the dill, remove the stem. Sprinkle over the gratin when it comes out of the oven. Serve hot.