Combine the onion powder, garlic powder, dried dill, 1 teaspoon salt and 1/2 teaspoon pepper in a small bowl. Put the wings in a large bowl and toss with 1 1/2 cups buttermilk, the onion slices and 1 teaspoon of the spice mix; cover and refrigerate 2 hours. Drain well, discarding the onion.
Preheat the oven to 425 degrees F. Brush a rimmed baking sheet with vegetable oil. Toss the wings with all but 1 teaspoon of the remaining spice mix and spread on the baking sheet. Roast until browned and crisp, about 45 minutes.
Meanwhile, make the ranch-chipotle sauce: Combine the mayonnaise, the remaining 2 tablespoons buttermilk, the vinegar, sugar, hot sauce and chives in a bowl. Add the reserved 1 teaspoon spice mix and season with salt. Cover and refrigerate until ready to serve.
Transfer the wings to a platter and serve with the sauce.
Photograph by Christopher Testani
Courtesy of Food Network Magazine