Preheat a deep-fryer with oil to 375 degrees F.
Bring a large pot of water to a boil over medium heat and add some salt. In small batches, add the cut potatoes and cook for 4 minutes, until they are soft. Carefully remove them from the boiling water and shock in a bowl of icy cold water. Drain and dry well with paper towels. Arrange the potatoes on a paper towel lined sheet tray and let air-dry. Repeat with each batch.
In a large bowl, whisk together 1 cup of the flour, the cornstarch, garlic powder, cayenne, smoked paprika, baking powder, and salt and pepper, to taste. Add the beer and whisk until incorporated. The batter will be slightly lumpy.
Add the remaining 1 cup of flour to a pie plate. In batches, dredge the fish fillets in the flour, then the beer batter. Let the excess batter drip off and gently lower into the fryer. Fry until golden brown and flip, roughly 2 to 3 minutes. Remove from the oil and arrange on a serving platter.
Add the blanched potatoes to the deep-fryer and fry until golden brown and crisp, about 5 to 6 minutes. Remove to paper towels to drain and sprinkle with kosher salt. Transfer to a serving bowl and serve with the fried fish and the Spicy Tartar Sauce.
Combine all the ingredients in a serving bowl. Cover with plastic wrap and let the flavors develop in the refrigerator for at least 1 hour before serving.