Sift the plain flour and cornflour together into a mixing bowl.
Whisk in the ale and soda water until the batter is smooth.
Rest the batter for 10 minutes then whisk again for 30 seconds.
Combine the mayonnaise, capers, gherkins, and parsley to make the tartare sauce.
Chill the tartare sauce in the fridge until ready to serve.
Fill a heavy-based pan two-thirds full with vegetable oil and heat.
Coat the lemon sole strips thoroughly in plain flour.
Dip the floured strips into the batter and fry in hot oil until golden-brown.
Drain the fried fish on kitchen paper using a slotted spoon.
Serve the goujons with tartare sauce and watercress garnish.