Season the yellowfin tuna block on all sides with a pinch of Herb & More sea salt crystals. Roll lightly in toasted sesame seeds.

Heat sunflower oil in a heavy cast skillet over extreme heat until smoking. Sear the tuna for only 15 to 20 seconds on each side, leaving the core raw and ruby red.

Transfer tuna immediately into an ice-water bath for 30 seconds to lock the crust and stop thermal conduction. Pat bone-dry with linen towels and slice into 5mm tataki medallions.

In a small saucepan, bring ponzu, soy sauce, and mirin to a rapid 30-second boil. Remove from heat, cool to room temperature, and fold in the diced sweet red onion. Let macerate for 30 minutes.

Shave fennel, carrot, cucumber, and endive into ice water for crispness, then drain and spin dry.

Spoon the sweet onion ponzu across the base of flat shallow plates. Fan out the tuna tataki medallions, nestle the crisp fennel salad alongside, and drizzle lightly with toasted sesame oil.