How to cook
Boil the potatoes without peeling them in salted water. Cool, peel and cut them into thin round slices. Steam boil the torn up head of broccoli for no more than 7-8 minutes.
In the dish you intend to use to bake the gratin, arrange the round slices of potatoes and broccoli florets, flavor with tarragon, sprinkle crumbled feta cheese (you can add blue cheese), distribute the butter and pour on the milk.
Sprinkle with breadcrumbs and grated cheese (you can substitute the cheese and breadcrumbs with Parmesan).
Bake in a preheated oven for about 15 minutes until golden brown.