Oven-roast the fries: Peel the potatoes while you preheat oven to 350F. Set aside one half of one of the peeled potates, then slice the other half, and the remaining potatoes, into long, 1/2-inch-wide oven-fry-sized strips. Put the cut potatoes in a mixing bowl and toss with the olive oil and a pinch of kosher salt. Transfer seasoned potatoes to a rimmed baking sheet and bake, 30-35 min, flipping or stirring the fries every 10 minutes or so to insure that all sides of the potatoes are browned.
When the fries are tender and golden-brown, remove them from oven and toss with the ground sumac.
Make the garlic-potato dip: Dice the half potato you set aside earlier, while you bring a small saucepan of salted water to a boil. Put the potato chunks in the boiling water and cook until soft, about 15 – 20 min. Remove cooked potato from the water, put in a small dish and roughly mash with a fork.
While the half potato is boiling, take the entire head of garlic apart, peeling the individual cloves and putting them in a blender or food processor. When the boiled, mashed potato is ready, add about 2 – 3 Tbsp of potato to the garlic in the food processor, along with an equal amount of water (2 – 3 Tbsp). Puree to make a thick garlic dip. Adjust the potato or water as necessary to achieve the desired consistency.
Serve immediately, with garlic-potato spread for optional dipping.