How to Make It
In a medium bowl, cover the sliced potatoes with the pickle brine and refrigerate overnight.
In a food processor, combine the garlic, lemon juice and salt and pulse until a coarse puree forms. Let stand for 10 minutes. Strain through a fine sieve set over a small bowl. Stir in the tahini, cumin and 3 tablespoons of water until smooth, then stir in the harissa and 1/2 tablespoon of the pickle brine. Season with salt.
Drain the potatoes and pat dry with paper towels. In a nonstick skillet, heat a thin layer of oil. Working in batches, fry the potatoes in a single layer over moderately high heat, turning occasionally, until they’re lightly browned and tender, about 5 minutes per batch. Drain and serve with the harissa tahini.