1. Preheat oven to 500°F with rack in upper third.
2. Mix oil with spices, 1 1/2 teaspoons salt, and 1 teaspoon pepper in a large bowl, then add chicken and onion, tossing to coat. Arrange chicken, skin side up, and onion in a 3-quart shallow baking dish.
3. Bake until chicken is just cooked through and skin is golden, about 30 minutes. Skim any fat from pan juices in dish.