Preheat the oven to 400°F.
Mix the mustard with lemon juice, sea salt, pepper, rosemary and a teaspoon of olive oil. Using a brush, marinade the salmon filet and allow to rest.
For the crust, put the pistachios in a mortar or plastic bag (or a cloth napkin) and grind with a hammer or another hard object.
Then mix the ground pistachios with the blueberries, breadcrumbs, grated cheese and the tablespoon of butter. Mix all the ingredients in the mortar until obtaining a wet dough.
Butter a baking dish and put the filets seasoned with salt, pepper and a pinch of nutmeg. Put a layer of the pistachio, blueberry, bread and cheese mixture over each filet, cover with aluminum foil and bake for 10-15 minutes, depending on the thickness of the filets. During the last 2-3 minutes, remove the foil so the pistachio crust becomes crunchy.
