Combine beer, oil, mustard, paprika, salt, pepper, and garlic in bowl, and stir to combine well.
Add onions, then in a sealed container, combine with chicken.
Let marinate in refrigerator for at least 6 hours, up to 24 hours.
Discard the marinade and grill on preheated grill until juices run clear, about 6 minutes on each side, depending on the thickness of your chicken.
Enjoy!!