Set aside to marinate for at least 20 minutes. For more flavor and a softer onion, allow to marinate for 2-4 hours, tossing a few times to make sure the marinade is evenly distributed.
Store in a tightly sealed container in the refrigerator for up to a week.
Notes
Substitutions:
White or yellow onions can be substituted for red onion.
For spicy sumac onions, add a pinch of red pepper flakes to the marinade.