Boil the potatoes: Place the whole, unpeeled potatoes in a large pot. Cover with cold water by at least an inch and add 1 tablespoon of salt. Bring to a boil over medium-high heat, then lower slightly and simmer for 18–20 minutes until fork-tender but firm.
Peel while warm: Drain the potatoes and allow them to cool just enough to handle. While still warm, peel away the skins using a small paring knife.
Slice and dress: Cut the warm potatoes into bite-sized cubes or wedges and transfer to a large mixing bowl. Immediately pour over the olive oil, lemon juice, sumac, salt, and black pepper. Gently toss to coat.
Add the fresh ingredients: Add the sliced red onion, chopped scallions, and parsley. Fold together gently with a large spoon to avoid mashing the potatoes.
Rest and serve: Let the salad sit at room temperature for 30 minutes before serving. This allows the potatoes to absorb all the flavors and the sharp onion to mellow.