In a small bowl, whisk together soy sauce, lemon juice, lime juice, mirin, brown sugar, and cayenne pepper. Divide sauce in half and reserve half the sauce for dipping.
Coat the tuna steaks in remaining ponzu sauce and marinate for at least one hour.
In a large nonstick skillet over medium-high heat, heat oil until shimmering. Remove tuna steaks from marinade, wipe off excess, and add to skillet without moving. Sear 1 to 2 minutes per side for rare.
Transfer to cutting board and let rest 5 minutes. Slice into ¼-inch slices and garnish with scallions. Serve with white rice and reserved ponzu sauce for dipping.
