Separate the egg. Stir egg yolk, flour, starch and beer until smooth and let it swell for about 15 minutes. Peel and finely dice the onion. Drain mustard gherkins and chop finely. Clean and wash the rocket and drain well. Put some small leaves aside for garnishing, chop the rest. Stir mayonnaise and sour cream until smooth. Stir in onion, cucumber and rocket. Season to taste with lemon juice, salt and pepper
Peel, wash and halve the potatoes. Cook in boiling salted water for about 20 minutes. Beat the egg white with 1/2 tsp salt until stiff and fold into the beer batter. Wash the fish, dab dry and cut into 4 pieces. Remove any bones. Season with salt and pepper. Heat oil in a large pan. Pull the fish through the beer batter and fry it in the hot oil for about 4 minutes on each side until golden brown
Wash the dill, dab dry. Pluck the pennants and chop finely, except for a little garnish. Drain the potatoes, let them steam briefly and sprinkle the dill over them. Drain fish on kitchen paper. Serve with remoulade and dill potatoes. Garnish with set aside rocket and dill. Serve with a fresh salad