Heat the oil in a 5-quart Dutch oven, over high heat, until it reaches 320 degrees F on a deep-fry thermometer.
Using a slicer with a wide blade or a chef's knife, slice the potatoes into chips.
Place in a large bowl with cold water.
In a large bowl, whisk together the flour, baking powder, salt, and green tea mix. Sift all the ingredients together to aerate the flour. Whisk in the beer until the batter is completely smooth and free of any lumps. Refrigerate for 15 minutes. Note: The batter can be made up to 1 hour ahead of time.
Drain potatoes thoroughly, removing any excess water and pat dry with paper towels. When oil reaches 320 degrees, submerge the potatoes, in batches, into the oil. Fry for 2 to 3 minutes until they are pale and floppy. Remove from oil, drain, and cool to room temperature.
Increase the temperature of the oil to 375 degrees F. Add the cooked fries and fry until crisp and golden brown, about 2 to 3 minutes. Remove and drain on roasting rack. Season with kosher salt while hot and hold in the oven. Allow oil to return to 350 degrees F.
Lightly dredge fish in flour. Working in small batches, dip the fish into batter and immerse into hot oil. When the batter is set, turn the pieces of fish over and cook until golden brown, about 2 minutes. Drain the fish on the roasting rack. Serve with malt vinegar and tartar sauce.