Preheat the oven to 425º F (218º C). Line a large sheet pan with foil; spray foil with cooking spray or olive oil.
In a small bowl, combine olive oil, lemon zest, lemon juice, salt, pepper, oregano, rosemary, garlic powder, garlic cloves, and red pepper flakes, if using. TIP: Make sure to zest your lemons before you squeeze them.
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 the chicken reads 165° F, and potatoes are tender. Halfway through the baking process, carefully flip each chicken and potatoes for even cooking.
OPTIONAL: In the last 3-5 minutes of the baking process turn your oven to broil mode and broil until the chicken is golden and the potatoes crisp and golden.
Garnish with fresh oregano and/or parsley, if using. Serve hot.