Preparation Place chicken on a cutting board and poke several holes through the skin with a sharp knife. Season with 1 tsp salt. Place chicken, skin side down, in a cold large cast-iron skillet and cook over medium heat, undisturbed, until chicken skin releases easily, about 5 minutes. Reduce heat to medium-low and continue to cook, lifting up once or twice to let hot fat run underneath, until skin is evenly golden brown and crisp, about 12 minutes. Meanwhile, whisk soy sauce, mirin, and sake in a small bowl. Transfer chicken skin side up to a plate. Wipe out skillet and increase heat to medium. Pour in soy sauce mixture and cook, stirring occasionally, until thickened slightly, about 4 minutes. Return chicken to pan, arranging skin side up, and cook, adding a splash of water if sauce looks too thick, until underside is cooked through, about 1 minute. Turn thighs skin side down and continue to cook until warmed through and coated, about 1 minute more. Transfer chicken to a cutting board and slice into 1/2" strips; set teriyaki sauce in skillet aside. Toss radishes, scallions, and vinegar in a small bowl. Season with remaining 1/2 tsp. salt. Place rice on a platter and arrange chicken over. Drizzle with reserved teriyaki sauce and top with radish salad.