Add the potatoes to a pot of cold, salted water. Bring to a boil, then reduce heat and simmer until just fork-tender, about 12-15 minutes. Drain and let cool slightly.

In a large mixing bowl, whisk together the olive oil, grated garlic, lemon juice, sumac, parsley, mint, green onion, salt, and pepper. Add the sliced onion and toss to coat.

Add the slightly warm potatoes to the dressing and gently toss to coat. Let the salad sit for a few minutes to absorb the flavors.

Taste and adjust seasoning if needed. Garnish with a little extra parsley and a sprinkle of sumac before serving.