How to Make It
In a bowl, cover the kombu with warm water and let stand until softened, 10 minutes. Drain and pat dry.
Spread the kombu on a large rimmed platter and rub with the lemon half. Set the tuna in a single layer on 2 pieces of the kombu and top with the lemon-rubbed side of the remaining kombu. Cover with plastic wrap and refrigerate for 24 hours.
In a bowl, whisk the 2 teaspoons of lemon juice with the orange zest and oil and season with smoked salt. Add the endives and toss. Peel off the kombu and transfer the tuna to plates. Sprinkle the fish with smoked salt and serve with the endive salad.
Kombu is available at some supermarkets, health-food stores and Japanese markets.