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