Steam the cod for 8 minutes, then take it off the bone.
Clean the mushrooms and remove the stalks. Poach the mushroom tops for 5 minutes in salted, boiling water with lemon in it. Drain and set aside.
Chop up the stalks with the shallots and sauté for 5 minutes in 20g butter.
Prepare the filling: in a bowl, mix together the cod, the chopped mushroom and the Beaufort cut into cubes.
Prepare the dough for the crumble: in a dish, knead the rest of the butter in small pieces with the breadcrumbs with your fingertips. Add the flour, a pinch of salt and work it into a coarse, grainy texture.
Place the filling into the mushroom tops, and cover with the crumble dough. Place in a greased ovenproof dish and cook for 10 minutes in an oven pre-heated at 180°C.