Remove the bones and skin from the cod (you can ask your fishmonger).
Cut the fillet in half crosswise, keep in the fridge.
Wash and halve the butternut, cut off the ends and scoop out the inside with a spoon and cut into cubes of about 2 cm. Place the pieces in a saucepan, add a pinch of coarse salt to the whole milk and fill 1 cm above the butternut pieces with clear water. Cook for about 30 minutes in a small broth, check that it is cooked with the tip of a small knife. Drain the butternut pieces and blend them with a blender. Add the double cream and the semi-salted butter, season to taste and set aside.
For the crumble in a bowl, knead all the ingredients until you obtain a crumbly paste, set aside.
Put the butternut mousseline in the bottom of a Pyrex container, place the cod fillet, previously salted and peppered, on top, sprinkle evenly over the crumble and bake at 180 degrees for 25 minutes.
During the cooking time, finely chop the chives.
After 25 minutes of cooking time, take the dish out and grate the truffle into thin slices, add the chopped chives and a drizzle of olive oil and serve immediately.