Fill a large bowl with ice water. Using a handheld slicer or food processor fitted with the slicing blade, slice the potatoes and place in the ice water.
In a medium bowl, dissolve 1 tsp. of the dry mustard in 1 tsp. cold water. Let sit 10 minutes. Stir in the mayonnaise, pickles and capers; add the pickle and caper juices to taste. Cover the tartar sauce and refrigerate.
In a medium bowl, mix the remaining 1 tsp. mustard, the flour, baking powder and 1 tsp. Old Bay.
In a 5-to 6-quart Dutch oven, heat the oil over medium until a deep-fry thermometer registers 365 degrees to 375 degrees . Drain the potatoes; pat dry. Working in batches, fry the potatoes, turning occasionally, until golden, 3 to 4 minutes per batch. (Adjust the heat to maintain temperature.) Using a slotted spoon, transfer chips to a paper towel/lined baking sheet. Season immediately with the remaining 1 tsp. Old Bay and salt.
Return the oil to 365 degrees . Whisk the beer into the flour mixture. Pat the fish dry; season with salt and pepper. Dust with flour, shaking off the excess. Coat in the batter. Working in batches, fry the fish, turning occasionally, until golden, about 5 minutes per batch. (Adjust the heat to maintain temperature.) Transfer to a paper towel/lined baking sheet; season. Serve the fish with chips, tartar sauce and lemon wedges.