Heat the oven to 425°F. Toss potatoes with 1 tablespoon oil, 1/2 teaspoon salt, 1/2 teaspoon paprika and 1/4 teaspoon pepper on an 18-by-13-inch rimmed sheet pan. Roast cut-side down for 15 minutes.
Pat thighs dry and rub with remaining oil, salt, paprika, garlic powder and pepper. Separate onion wedges into a few thick pieces.
Turn the potatoes and tuck onion wedges among them. Set thighs skin-side up in open spaces on the pan, keeping the pieces in one layer.
Roast for 35 to 40 minutes more, turning exposed potatoes once, until their centers are tender and the thickest chicken meat reaches 175 to 185°F.
Rest for 5 minutes, then serve each thigh with one-quarter of the roasted potatoes and onions.