Bake the washed, unpeeled potatoes wrapped in aluminum foil until tender. Peel them while hot and keep warm.
Portion the cod fillet into 180 g slices, keeping the skin intact. Set aside.
Finely julienne the white parts of the leeks. Fry at around 170°C until lightly colored. Season with salt and freshly ground pepper. Set aside.
Reduce the veal jus, then whisk in the butter off the heat. Season with salt and pepper. Set aside.
Season the cod portions with salt and pepper, coat lightly with olive oil, and sear skin-side down. Finish cooking in the oven.
Meanwhile, mash the warm potatoes in a saucepan with a fork, incorporating the butter, salt, and pepper to taste.
On hot plates, shape the mash into 10 cm circles in the center. Top with the cod portions (skin-side down), add a small dome of fried leeks on each, and surround with the sauce—stir in the truffle juice at the very last moment.
Finish by dusting with paprika and poppy seeds around the plate, and sprinkle finely chopped chives.
