Start with your ahi tuna steaks. Grab some paper towels and pat them dry. This helps the oil and spices stick better. Place them on a plate, then drizzle a little olive oil and soy sauce over both sides. Rub it in with your fingers, like giving the tuna a quick massage. This adds flavor and gets the tuna ready for the coating. It only takes a minute, and it makes a significant difference.
In a shallow bowl or plate, mix your black sesame seeds, white sesame seeds, coarse black pepper, salt, garlic powder, and a pinch of ground ginger if you are using it. Stir it up so everything is even. Now, take each tuna steak and press it into the mixture, one side at a time. Really push it in with your hands so the coating sticks well. Do not forget the edges! This crust is what gives the tuna its crunchy, flavorful outside.
Get your frying pan or cast-iron skillet and put it on medium-high heat. Let it get nice and hot. You can assess it by sprinkling a drop of water in the pan. If it sizzles right away, you are good to go. Add a small splash of vegetable or olive oil, just enough to coat the bottom lightly. You do not want the tuna swimming in oil, just enough to sear it nicely.
Carefully place the coated tuna steaks in the hot pan. Let them sear for about 30 to 45 seconds. On one side you will see the edges turn golden. Flip them with tongs or a spatula and sear the other side for another 30 to 45 seconds. The goal is a crispy outside with a pink, cool center. Do not cook too long, or the tuna will dry out. It is quick, so stay focused!
Once seared, take the tuna out of the pan and set it on a plate. Cover it with plastic wrap or pop it into a container, then stick it in the fridge for 2–3 hours. Chilling firms up the tuna, making it easier to slice thinly, like you see in restaurants. When it is ready, use a sharp knife to cut it into thin, sashimi-style slices. It looks so professional!
In a small bowl, mix soy sauce, hoisin sauce, rice vinegar, and a few drops of sesame oil. Stir it well until it is smooth. For a fresh touch, toss in some chopped green onions or cilantro. Drizzle a little sauce over your sliced tuna and serve the rest on the side for dipping. This sauce is what makes every bite so tasty!