Heat the oven to 425°F. Stir together the butter, 2 tsp. of the ginger, 2 tsp. of the lemon zest, half the garlic, the coriander, the cumin, 1/2 tsp. salt, and 1/4 tsp. pepper in a small bowl. In another small bowl, stir together the cilantro, the remaining ginger, lemon zest, and garlic to make the gremolata garnish. Set aside.
Smear the butter mixture under the skin of the chicken thighs. Arrange the chicken skin side up on one end of a large rimmed baking sheet. Sprinkle lightly with salt. At the other end of the baking sheet, toss the potatoes and lemon wedges with 1/2 tsp. salt and 1/4 tsp. pepper and spread into a single layer. Roast for 20 min. Baste the chicken with the pan juices using a brush, and flip the potatoes and lemon with a spatula (they may be a little stuck and need loosening). Continue to roast, basting the chicken and flipping the potatoes every 10 min., until the chicken and potatoes look crisp and deeply golden, about 30 min. more.
Stir the potatoes and lemon, transfer to a serving bowl with a slotted spoon, and gently stir in half of the cilantro gremolata. Baste the chicken and transfer with tongs to a serving platter, sprinkle with the remaining gremolata, and serve hot.
Try a crisp unoaked Sauvignon Blanc like the 2005 Kim Crawford, Marlborough ($17) or the 2004 Frog’s Leap, Napa ($16.50).