Stir onion, honey, brown sugar, soy sauce, butter, mustard powder, and garlic together in a saucepan over medium heat; cook until hot, about 5 minutes.
Spread chicken wings in an even layer into a shallow baking dish; pour sauce over the wings.
Marinate wings in refrigerate 1 hour to overnight.
Preheat oven to 350 degrees F (175 degrees C).
Bake the chicken wings, turning occasionally, until no longer pink at the bone and the juices run clear, about 90 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).