Roll tuna steaks in cracked black pepper. Heat 1/2 tablespoon sesame oil in a small nonstick skillet over high heat. Sear tuna for 2 minutes. Turn and sear second side 2 minutes. Remove to a cutting board and thinly slice.
Mix soy sauce, remaining tablespoon sesame oil, garlic, and five-spice powder together in a small bowl. Serve sliced tuna and spoon sauce on top. Sprinkle with grated daikon radish.