Transfer red onion slices to a medium bowl and toss with the sumac, chile flakes, salt and pepper. Massage the spices into the onion with your hands to soften and set aside.
Squeeze the seeds from the quartered tomatoes and roughly chop. Add them to bowl with onions along with the parsley, lemon juice, olive oil and beans tossing to combine with spoon.
Serve with grilled meats, lahmuchan or sprinkled with crumbled feta cheese as a side salad.
Notes
Although navy beans are the most popular for this salad, any variety of cooked white beans may be used. Cannellini, Great Northern, etc.