1/3 cup mirin
2 tablespoons soy sauce
1 tablespoon toasted sesame oil
4 (six-ounce) tuna steaks
1 teaspoon ground cumin
1 teaspoon five-spice powder
1 teaspoon sugar
1/2 teaspoon sea salt, or to taste
1/2 teaspoon freshly cracked black pepper, or to taste
Lime wedges (optional)
In a shallow bowl, combine the mirin, soy sauce and sesame oil. Add the tuna and turn to coat. Cover and refrigerate for 30 minutes.
Preheat the grill.In a small bowl, combine the cumin, five-spice powder, sugar, salt and pepper. Set aside.
Transfer the tuna to a platter, reserving the marinade. Sprinkle the spice mixture over both sides of the tuna. Grill the tuna, turning once, to the desired degree of doneness, about 4 minutes per side for medium. Transfer the tuna to a cutting board; set aside to rest for 3 minutes.
Meanwhile, pour the reserved marinade into a small saucepan. Over medium heat, bring the mixture to a boil and cook for 1 minute. Remove from the heat; set side.
Cut the tuna into slices or wedges, arrange on serving plates and drizzle with the marinade sauce. If desired, serve with lime wedges.
The tuna and reduced marinade can be spooned over mildly peppery greens, such as arugula or mizuna, or rice and steamed green beans.