Step 1

To make the gratin, heat butter in a heavy pan, add leeks, and season with salt and pepper. Cook over low heat, stirring now and then, until softened and no longer bright green, about 15 minutes.

Step 2

While the leeks are cooking, place chicken in a bowl with mustard, worcestershire sauce, cayenne, thyme, and salt and stir to combine. Allow to sit for 10 minutes while the leeks cook.

Step 3

Remove cooked leeks from heat and mix in cream or chicken broth. Spread evenly in the base of a large, shallow baking dish, then make the crust (recipe below).

Step 4

Arrange chicken thighs on top of the leek mixture, then cover with the crust. The gratin can be prepared in advance to this stage and chilled for several hours until needed.

Step 5

Make Crust (Recipe Below)

Step 6

When ready to cook, preheat oven to 350° F. Bake until chicken is fully cooked through and crumbs are golden (about 1 hour).

Step 7

To make the crust, break bread into rough chunks, removing crusts if they are very hard, and pulse in a food processor until coarse crumbs form. Measure out 2 heaped cups and freeze any remaining crumbs for future use. Place garlic, parsley, rosemary, anchovies, butter, and lemon zest in a food processor and whizz to chop finely. Add breadcrumbs and parmesan and pulse to just combine. This crust will keep in a container in the fridge for up to a week or can be frozen.