Golden leek gratin

Butter, unsalted	6 tablespoons
Shallots, minced	2 tablespoons
Leeks, white and tender green parts, chopped	3 pounds
Salt	½ teaspoon + ½ teaspoon
Pepper, freshly ground	½ teaspoon + ½ teaspoon
Milk, whole	2 cups + extra
Flour	3 tablespoons
Cayenne pepper	¼ teaspoon
Gruyere cheese	½ cup
Bread crumb, fresh	½ cup

1. Preheat oven to 400°F. 
2. Melt 2 tablespoons of butter in a large frying pan over medium-high heat. 
3. Add the shallots, leeks, ½ teaspoons of salt, and ½ teaspoons of pepper. 
4. Reduce heat to medium, and cook, stirring often, until the leeks are translucent and soft, about 15 minutes; set aside. 
5. In a small saucepan over medium heat, warm the milk until small bubbles form around the edges of the pan; cover and remove from heat. 
6. In a saucepan over medium-high heat, melt 3 tablespoons of butter. 
7. Remove from the heat, and whisk in the flour, cayenne, ½ teaspoons of salt, and ½ teaspoons of pepper. 
8. Return the pan to medium-low heat, slowly whisk in the hot milk, and simmer, stirring until the sauce thickens.
   If the sauce is too thin, raise the heat; if it is too thick, add more milk. 
9. Stir into the leek mixture. 
10. Pour the mixture into a baking dish. 
11. Sprinkle evenly with the cheese and bread crumbs, and dot with the remaining butter. 
12. Bake until the top is golden and the gratin is bubbly, about 20 to 30 minutes.

Recipe from The World Kitchen by William-Sonoma.