make the sauce
Add 2 tablespoons of oil into a small saucepan and fry the onions over medium heat until they are soft. Add the ginger and garlic and continue frying until they are very fragrant and browned all the way through. Add the curry powder, ground coriander, and flour and fry until saturated with oil and fragrant. Add the ketchup and stock then whisk, cooking until the sauce thickens up just a bit.
fry the chips the first time
Add about 3/4" of oil to a heavy bottomed pan (cast iron ones work great), and heat over medium heat until it reaches 325 degrees F. Carefully lower some potatoes into the oil without overcrowding the pan. Fry until they are limp but not brown. Transfer to a paper towel lined plate and fry the rest of the potatoes.
for beer battered fish
Mix the flour, cornstarch, paprika, onion powder, salt and pepper in a bowl. Put about 1/2 C of the mixture into a shallow pan or bowl and dust the cod with the mixture.
When the chips are done, raise the temperature of the oil to 375 degees F. Add 3/4 tsp of baking powder to the unused flour mixture in the bowl then add about 1 C of cold beer. Gently stir until just combined (it's okay if there are still a few lumps), and add more beer if the mixture seems too thick (should be like thick pancake batter). Dip the cod in the batter, coating and allowing any excess to drip off and place in the hot oil.
Fry until golden brown on one side then flip and fry until golden brown on the second side. Transfer to paper towel lined wire rack and repeat until all the cod is fried. Sprinkle with sea salt.
fry the chips the second time
Add a handful of potatoes into the hot oil and fry until crisp and golden brown. Repeat until all the chips are fried. Sprinkle with sea salt.
Plate a mound of fries, top with a piece of fried cod and serve with curry sauce and a wedge of lemon or malt vinegar.