Wash and pat dry the fish fillets with kitchen paper. Sprinkle salt & pepper powder and keep aside.
In a deep bowl combine the flour, paprika,garlic powder & celery salt & salt. Add an egg and beat the mixture. Then add the cold beer till to get a thick batter consistency.
To make the Tartar sauce. Add the finely chopped onions & gherkins to the mayonnaise. Then add the mustard sauce, lime juice and pepper & salt as per your taste. Whisk it with a fork to make a fine sauce.
Heat oil in a frying pan and dip the fish fillet is the beer batter & fry in the hot oil. Each side will cook in 2 mins each side. Once you get that golden brown color remove from oil and let all the excess oil absorb on a kitchen paper.