How to Make It
Pat chicken dry with paper towels. Sprinkle chicken evenly with salt and let stand in the refrigerator for at least 30 minutes (or up to overnight). Bring chicken back to room temperature before cooking.
Preheat oven to 425°.
Rub skin side of the chicken thighs evenly with 1 tablespoon olive oil. Place thighs, skin side down, in a single layer in a medium cast-iron skillet over medium heat. Let fat render for about 5 minutes, then increase heat to medium-high. Cook until skin is golden brown and beginning to crisp, another 8 minutes (do not move chicken until the edges start to turn golden). Turn thighs over and cook other side for about 3 minutes; transfer to a plate.
Add the grapefruit slices, zest, and rosemary to the cast-iron skillet, and cook until citrus is caramelized, about 3 minutes on each side. Return thighs to the pan, skin side down and transfer to oven. Bake at 425° for 5 minutes, or until a thermometer inserted in thickest part of chicken thigh reads at least 170 °.
Toast walnuts in a small skillet over medium heat until golden brown and fragrant, about 6-8 minutes. Combine walnuts, olives, feta, vinegar, and remaining 2 tablespoons olive oil in a bowl. Top chicken with walnut mixture and serve immediately.