01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or oil.
02 - Coat chicken thighs with olive oil, then season evenly with kosher salt and black pepper. Arrange in the prepared baking dish, skin-side up.
03 - In a small saucepan, combine 2 sliced peaches, ketchup, apple cider vinegar, honey, brown sugar, Dijon mustard, Worcestershire sauce, smoked paprika, garlic powder, and onion powder. Bring to a boil over medium heat, then reduce to a simmer and cook for 10 minutes until the peaches are very soft. Blend with an immersion blender or transfer to a standard blender and purée until smooth.
04 - Pour half of the peach BBQ sauce over the chicken thighs, spreading to coat each piece evenly. Reserve the remaining sauce for serving.
05 - Arrange fresh peach slices around and on top of the chicken thighs in the baking dish.
06 - Bake uncovered for 35 to 40 minutes, until the chicken reaches an internal temperature of 165°F and the skin is golden and caramelized. For additional browning, broil for 2 to 3 minutes at the end.
07 - Remove from the oven and let rest for 5 minutes. Garnish with chopped cilantro or parsley if desired, and serve with the reserved peach BBQ sauce drizzled on top or alongside.