Preheat the oven to 400°F (200°C) with a wire rack on a lined baking sheet. Whisk together the flour, garlic powder, onion powder, smoked paprika, thyme, oregano, celery salt, black pepper, and salt in a shallow bowl. Beat the eggs and milk in a second bowl. Pat the chicken dry, dip each piece in the egg wash, then press firmly into the flour mixture on all sides.

Heat the 3 tbsp oil in a large skillet over medium-high heat. Sear the coated chicken in batches, skin-side down first, for 3 to 4 minutes per side until pale golden, then transfer to the wire rack.

Bake the chicken at 400°F (200°C) for 30 minutes, until the internal temperature reaches at least 160°F (71°C).

While the chicken bakes, stir together the BBQ sauce, apple cider vinegar, brown sugar, and Worcestershire sauce in a small saucepan over medium-low heat. Simmer for 5 minutes until slightly thickened, then remove from heat.

Switch the oven to broil on high. Brush a generous coat of glaze over all the chicken pieces. Broil for 3 minutes, brush on a second coat, then broil for another 2 to 3 minutes until bubbling and deeply caramelized.

Transfer to a serving platter, brush on any remaining glaze, and scatter sliced scallions over the top before serving.