Pat the ahi tuna dry with paper towels. Trim any sinew so you have a clean rectangular loin about 1 to 1 1/2 inches thick. Lightly season all sides with kosher salt and freshly ground black pepper (use the quantities listed).

Make marinade/sauce

In a small bowl whisk together the low-sodium soy sauce, rice wine vinegar, mirin, sesame oil, grated ginger, and minced garlic until combined. Reserve about 1 tablespoon of this mixture for serving; the rest will be used to briefly flavor the tuna and as a light finishing sauce.

Marinate briefly

Place the tuna in a shallow dish and pour about 1 tablespoon of the marinade over it, turning to coat all sides. Let rest at room temperature for 5 minutes (do not marinate longer than 15 minutes for best texture and to avoid over-salting). Pat any excess marinade off before crusting.

Crust & Sear

Coat with sesame

On a plate spread the mixed sesame seeds evenly. Press each side of the tuna lightly into the seeds so the top, bottom, and short edges get an even, thin coating. Do not press so hard that the seeds fall off, but ensure an even coverage.

Heat the pan

Heat a heavy skillet or cast-iron pan over high heat until very hot (about 2–3 minutes). Add 1 tablespoon neutral oil and swirl to coat. The oil should shimmer but not smoke heavily.

Sear quickly

Place the sesame-crusted tuna in the hot pan. Sear for 20–30 seconds per side for a rare center (1–1 1/2 inch thick loin). For a slightly more done edge, sear the short edges for 10–15 seconds each. Use tongs to turn gently. The sesame seeds should be golden and fragrant but the center should remain pink/rare.

Rest and slice

Remove the tuna to a cutting board and let rest 1 minute. Using a very sharp knife, slice the tuna across the grain into 1/4 inch (about 6 mm) slices. Wipe the knife clean between cuts if seeds stick.

Assemble & Serve

Plate wakame

Divide the wakame salad between serving plates as a bed or on the side (use the full 1 cup across servings). If your wakame is very salty, rinse briefly and drain before plating and pat dry.

Arrange tuna

Fan the sesame-crusted tuna slices over or beside the wakame. Drizzle the reserved tablespoon of marinade/sauce lightly over the tuna slices and around the plate. If desired, finish with a tiny drizzle (about 1/2 tsp total) of toasted sesame oil for aroma.

Add accompaniments

Place a small dollop of wasabi paste on each plate or serve on the side, and add a portion of pickled ginger. Scatter the thinly sliced scallions over the tuna and wakame. Garnish with microgreens or cilantro if using, and serve lemon or lime wedges alongside for optional brightening.

Service notes

Serve immediately while the seared exterior is warm and the center remains rare. Provide soy sauce or extra reserved marinade on the side for dipping. Leftover tuna is best eaten the same day and should be chilled uncovered briefly before storing.