Mix up the liquid ingredients and seasonings in a zip-top storage bag or a storage container. You may need to heat the mixture slightly if adding brown sugar to get it to fully dissolve
Add the Flank steak. Add up to ¼ cup of water if needed to cover the steak as much as possible. If the top surface of the steak remains exposed, flip the meat two to three times during the marinating time to ensure even absorption. Freeze the steak in the marinade after just two to three hours of marinating if desired
Marinate the meat for up to 48 hours, then cook with your preferred method.
Discard the marinade due to bacteria from the raw meat. If you want to turn the Flank steak marinade into a sauce, make sure to bring it to a boil for 10 minutes before thickening or reducing it. Don't store the marinade for reuse because it isn't considered a safe practice