Put potatoes in a large pot filled with water. Add 1 or 2 teaspoons salt into the pot. Boil the potatoes until you can easily insert a fork in them. It takes about half an hour.
Put finely sliced onions in a large bowl. Sprinkle sumac and salt on it. Mix it well with your hand, squeezing the onion slices so that they release juice and get the color of sumac. This is a way of removing their bitterness.
Drain potatoes, wait for a few minutes in cold water. Peel and dice the potatoes into the bowl.
Toss in green onions, avocado and eggs.
Add in olive oil, lemon juice and black pepper and stir until combined well.
Taste and add extra salt or lemon juice if needed.