Preheat oil in the deep fryer to 350°F (180°C). Place a wire rack on a baking sheet. Preheat the oven to 200°F (100°C) to keep fries warm while frying fish.
Soak potatoes in warm water for about 5 minutes to remove any excess starch. Drain and pat potatoes dry with a clean cloth.
Fry potatoes for about 5 minutes or until tender and lightly browned. Drain fries and place on the rack. Let cool.
Increase oil temperature to 375°F (190°C). Place potatoes back in the fryer and fry for 3 to 4 minutes or until golden brown. Remove from fryer and drain on the rack. Season with salt and set aside in the oven. Fry parsnips until tender and golden brown. Drain on the rack and set aside with fries in the oven.
In a bowl, combine flour, baking powder and salt. Gradually add beer, whisking until smooth. Season pieces of fish with salt and pepper.
Dip each piece in batter to coat well. Place pieces in fryer basket. Fry for 4 to 5 minutes, shaking basket to prevent them from sticking to the bottom. Remove fish and drain on the rack. Set aside in the oven while cooking remaining fish pieces.
Dip pickles in remaining batter. Drain slightly and fry until crispy.
Serve fish, fries and pickles with Sorrel Cream Sauce (see recipe) or mayonnaise.