Place 1 1/2 teaspoons onion powder, 1 1/4 teaspoons kosher salt, 1 teaspoon garlic powder, 1 teaspoon paprika, 1/2 teaspoon dry mustard powder, and 1/2 teaspoon black pepper in a large zip-top bag or large bowl and stir to combine.
Pat 8 chicken drumsticks dry with paper towels. Place in the spice mixture and toss until the chicken is evenly coated. If baking immediately, let sit at room temperature for at least 20 minutes while you heat the oven. Otherwise, cover or seal the bag and refrigerate up to overnight.
Arrange a rack in the middle of the oven and heat the oven to 400°F. Line a baking sheet with aluminum foil, then fit it with a wire rack. If the chicken has been refrigerated, let it sit at room temperature while the oven is heating.
Place the chicken skin-side up on the wire rack. Roast until the chicken is browned, crisp, and an instant-read thermometer inserted into the thickest part of the drumstick not touching the bone registers at least 165°F, about 30 minutes.