Preheat oven to 425°F.
Line a very large rimmed baking sheet with foil (for easy clean-up) and spray with nonstick cooking spray or brush with olive oil. If you don't have an extra large baking sheet, you might need to use two separate smaller baking sheets to accommodate both the potatoes and the chicken.
Arrange lemon slices and onion slices on half of the prepared baking sheet. Pat chicken dry; arrange (skin-side up) on top of the lemons and onions.
In a small bowl, whisk together lemon juice, vinegar, honey, olive oil, garlic, and oregano. Set aside half of the marinade in a bowl. Spoon or brush the remaining sauce over the chicken. Season chicken liberally with salt and pepper.
Roast chicken for 45 minutes – 1 hour, or until the meat reaches an internal temperature of 165°F. If the chicken starts to get too dark on top while it’s still cooking, shield loosely with foil.
When there are about 30 minutes left in the chicken's cooking time, toss the potatoes with the remaining dressing. Spread the potatoes in a single layer on the other half of the baking sheet (or on a separate baking sheet). Season with salt and pepper.
Return the pan to the oven for 25-30 more minutes, or until the chicken reaches an internal temperature of 165°F and the potatoes are fork tender and crispy.