Preheat the oven to 450 degrees F. Place an empty baking sheet in the oven to heat.
Cut the potatoes into 1/2-inch wedges. Toss with 2 tablespoons of the oil and season with salt and black pepper. Spread the potatoes evenly on the heated baking sheet, return to the oven and roast until dark golden at the edges and slightly crisp, about 20 minutes.
Meanwhile, combine the mayonnaise, harissa, cornichons and a squeeze of lemon juice in a small bowl. Cover and refrigerate.
Whisk together the chickpea flour, cornstarch, cayenne, 1 tablespoon salt and 1/2 teaspoon black pepper in a shallow dish. Whisk the eggs with 1/4 cup water in another dish. Cut the fish into 2 by 1-inch planks. Dredge the fish in the chickpea flour mixture, dip in the egg and then dredge again in the flour mixture. Set aside on a baking sheet to rest.
Heat the remaining 3/4 cup oil in a 10-inch cast-iron skillet over medium-high heat until it shimmers. Add the fish in batches and fry until golden brown and cooked through, about 2 minutes per side. Remove to a cooling rack or paper towels to drain. Season with salt and black pepper.
Serve the fish with the potatoes, tartar sauce and lemon wedges. Garnish with parsley, if desired.
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