Preheat oven to 400°F with rack about 8 inches from heat. Combine onion powder, salt, thyme, pepper, cinnamon, nutmeg, 2 tablespoons of the parsley, and 2 teaspoons of the oregano in a mini food processor; pulse several times until well blended. Add oil; pulse until combined.

Place chicken and lemons on a rimmed baking sheet; rub chicken evenly with 1⁄4 cup of the herb mixture. Toss potatoes with remaining 3 tablespoons herb mixture. Arrange potatoes around chicken and lemon slices. Roast in preheated oven until a thermometer inserted in thickest portion of chicken registers 155°F and potatoes are tender, about 30 minutes

Increase oven temperature to broil. Broil until skin is browned and crisp, about 5 minutes. Remove from oven, and let stand 5 to 10 minutes. Sprinkle with tomato, olives, feta, and remaining 2 tablespoons parsley and 1 teaspoon oregano.