Begin by cutting the red onion in half from stem to root. Then, thinly slice each half, cutting from the stem to the root.
Transfer the sliced onions to a mixing bowl. Add sumac, olive oil, lemon juice, and salt. Toss the mixture thoroughly using your hands or with the aid of two forks to ensure the onions are well-coated with the seasonings.
Transfer the seasoned onions, along with all the juices, into a jar. Allow them to marinate for at least 30 minutes before consuming to enhance the flavors. Store any leftovers in the refrigerator. They will keep well for up to 3 days.