Combine the ingredients: In a medium bowl, combine the onion slices with vinegar, sumac, salt, parsley, and mint.
Toss everything together: Use your hands to toss everything together, massaging gently.
Let it sit: Cover and refrigerate for 30 minutes to 1 hour.
Serve: Use immediately or store in the fridge in a tightly covered jar for up to 1 week.
