Step 1 To make the onion marinade, in a blender combine the onion, garlic, oil, and water and blend until smooth. Put the skirt steak in a large resealable plastic bag or baking dish and pour in the onion mixture, coating both sides. Cover or wrap the meat well so the onion scent does not permeate your refrigerator; chill for at least 4 hours and up to 24 hours.