6 Tbs. (3/4 stick) unsalted butter
2 Tbs. minced shallots
6 to 8 leeks, about 3 lb. total, white and light green portions,
1 tsp. salt
3/4 tsp. freshly ground white pepper
2 cups milk, plus more as needed
3 Tbs. all-purpose flour
1/4 tsp. cayenne pepper
1/2 cup shredded Gruyère cheese
1/2 cup fresh bread crumbs
Preheat an oven to 400°F.
In a fry pan over medium-high heat, melt 2 Tbs. of the butter. Add the shallots, leeks, 1/2 tsp. of the salt and 1/2 tsp. of the white pepper. Reduce the heat to medium and cook, stirring often, until the leeks are translucent and very soft, about 15 minutes.
In a small saucepan over medium heat, warm the 2 cups milk until small bubbles appear around the edge of the pan. Cover and remove from the heat.
In a saucepan over medium-high heat, melt 3 Tbs. of the butter. Remove from the heat and whisk in the flour, the remaining 1/2 tsp. salt, 1/4 tsp. white pepper and the cayenne. Set the pan over medium-low heat and cook, stirring often, for 2 minutes. Slowly whisk in the hot milk and simmer, stirring, until the sauce thickens, about 15 minutes. If the sauce is too thin, increase the heat; if it is too thick, whisk in a little more milk. Then stir in the leek mixture.
Pour the mixture into a baking dish or a gratin dish. Sprinkle evenly with the cheese and bread crumbs, then dot with the remaining 1 Tbs. butter. Bake until the cheese and bread crumbs are golden and the gratin is bubbly, 20 to 30 minutes. Remove from the oven and serve. Serves 6.