Preheat oven to 425° F. Line a 15x10 inch baking pan with foil; coat foil with cooking spray.
In a small bowl, combine olive oil, lemon juice, lemon zest, salt, pepper, oregano, rosemary, garlic powder, garlic cloves, and red pepper flakes, if using.
Arrange potatoes and chicken on the prepared baking sheet. Drizzle oil mixture over the chicken and potatoes; mix to coat. Place lemon slices and onion around the chicken, if using.
Roast for 35- 40 minutes, or until a thermometer inserted in chicken reads 165°, and potatoes are tender. Halfway through the baking process, carefully flip each chicken and potatoes for even cooking.
OPTIONAL: The last 3-5 minutes of the baking process turn your oven to broil mode and broil until chicken is golden and potatoes crisp.
Garnish with fresh oregano and/or parsley, if using. Serve hot.