Gather all the ingredients.
Grate the ginger and collect 1 tsp ginger (grated, with juice). Slice 1 green onion/scallion thinly and set aside.
Combine the sauce ingredients in a small bowl: the green onions, the grated ginger, 3 Tbsp ponzu, 2 tsp toasted sesame oil, 1 tsp soy sauce, and 1 tsp toasted white sesame seeds. Set aside.
Heat a nonstick frying pan. When the pan is hot, add 2 Tbsp neutral oil. When the oil is hot, add ½ lb sashimi-grade yellowfin/ahi tuna and sear it 30 seconds on each side.
When all sides are seared, remove from the heat and let it cool. Slice the tuna into ¼-inch (6-mm) pieces. Pour the sauce on the Tuna Tataki and serve with ½ lemon and Korean chili thread.
You can keep the leftovers in an airtight container and store in the refrigerator for a day.
