Pat the tuna steaks completely dry with paper towels. In a shallow dish, combine the soy sauce and sesame oil. Place the tuna in the mixture and turn to coat all sides. Let it marinate at room temperature for 10 minutes, flipping once halfway through.
On a plate, mix together the white sesame seeds, black sesame seeds, and garlic powder. Remove the tuna from the marinade and press each steak firmly into the sesame mixture, coating all sides generously.
In a small bowl, whisk together the soy sauce, rice vinegar, sesame oil, honey, grated ginger, and sriracha until the honey dissolves. Set the dipping sauce aside.
Preheat the air fryer to 400 degrees F. Lightly spray the basket with cooking spray. Place the tuna steaks in the basket with space between them.
Air fry for 3 minutes, then carefully flip using a thin spatula to keep the sesame crust intact. Cook for another 2 to 3 minutes for a beautifully seared outside with a rare to medium-rare center. Add 1 to 2 extra minutes if you prefer medium.
Let the tuna rest for 2 minutes, then slice against the grain into half-inch strips. Fan the slices on a plate and serve alongside the dipping sauce.