Tuna Tataki with Ponzu

Steps

1. Make the ponzu: finely grate the ginger. In a bowl, mix the soy sauce, 2 tablespoons of lemon juice, 1 tablespoon of lime juice, the mirin, rice vinegar, toasted sesame oil and grated ginger. Let it steep in the fridge for at least 15 minutes.
2. If the loin is thick or uneven, trim it into two rectangular blocks about 1 1/2 inches (4 cm) square, so it sears evenly. Pat the tuna very dry with paper towels on all sides and keep it cold until cooking.
3. Mix the white and black sesame seeds with the cracked pepper on a plate. Brush the tuna with 1 teaspoon of the grapeseed oil, season with half the flaky salt, then roll it in the sesame, pressing so every side is coated, ends included.
4. Heat a cast-iron or carbon-steel skillet over high heat for 3 minutes. Add 1 tablespoon of grapeseed oil: it should shimmer and give off a faint wisp of smoke.
5. Lay the tuna in the pan and sear 20 to 30 seconds on each of the four long sides, then 10 seconds on each end, holding it with tongs. The sesame should be golden and only a 1/8 inch (2 to 3 mm) gray band should show on the cut side; the center stays red and raw. If the sesame blackens before 20 seconds, lower the heat slightly.
6. Transfer the tuna to a plate and refrigerate for 15 minutes, uncovered: the cooking stops and the flesh firms up for slicing. Meanwhile, finely grate the daikon and squeeze it gently in your hands to remove excess water, slice the radishes paper-thin on a mandoline, and slice the scallions.
7. Slice the tuna with a long, sharp knife into 1/3 inch (7 to 8 mm) slices, in a single pulling stroke rather than sawing, and wipe the blade between slices to keep the edges clean.
8. Fan the slices on chilled plates, add a small mound of grated daikon and the radish slices, and scatter the scallions and a pinch of the remaining flaky salt on top. Serve the ponzu on the side in small dishes, or spoon it around the slices, never over them, to keep the crust crisp. Cut the rest of the lime into wedges to serve alongside.