To make the Provencal crust, break the bread into rough chunks (remove the crusts if they are very hard) and pulse in a food processor until coarse crumbs form. Measure out two heaped cups and freeze any remaining crumbs for future use.
Place garlic, parsley, rosemary, anchovies, butter and lemon zest in a food processor and whiz until they're finely chopped.
Add breadcrumbs and parmesan and pulse to just combine (the Provencal crust will keep in a container in the fridge for up to a week or can be frozen).
To make the gratin, heat butter in a heavy pan, add leeks and season with salt and pepper.
Cook over a low heat, stirring now and then until the leeks are soft and are no longer bright green (this should take about 15 minutes).⏱ 15 min
While the leeks are cooking, place the chicken in a bowl with the mustard, Worcestershire sauce, cayenne pepper, thyme and salt and stir to combine.
Allow to sit for 10 minutes while the leeks cook.⏱ 10 min
Remove cooked leeks from the heat and mix in the cream or chicken stock. Spread evenly in the base of a large, shallow baking dish.
Arrange chicken thighs on top then cover with Provencal crust. The gratin can be prepared in advance to this stage and chilled for several hours until needed.
When ready to cook, preheat oven to 180°C and bake until the chicken is fully cooked through and the crumbs are golden (about 1 hour).⏱ 60 min