Preheat the oil in a deep fryer to 250°F (120°C). Line a baking sheet with paper towels.
In a large bowl, rinse the potatoes under cold running water until the water runs clear. Let soak for 10 minutes. Drain and pat dry with a clean dishcloth.
Blanch half of the potatoes at a time in the oil for 5 minutes. Drain and place on one half of the baking sheet. Repeat with the remaining potatoes. Set aside.
Increase the temperature of the deep fryer to 350°F (180°C). Preheat the oven to 200°F (95°C).
Fry half of the blanched potatoes in the oil for 2 minutes or until golden and crispy. Drain well. Place in a metal bowl and immediately season with salt and half of the sumac. Mix well. Transfer to a baking sheet and keep warm in the oven, uncovered. Repeat with the remaining blanched potatoes and sumac.
Serve the fries with harissa mayonnaise, if desired.