For the batter, mix flour, starch, baking powder and 1 teaspoon salt. Stir with lemon juice and about 180 ml water until smooth. Wash the fish, dab dry and cut into portion pieces.
For the pea puree, peel and finely dice the onion. Fry in hot butter. Add 200 ml water, stock and frozen peas. Bring to the boil and simmer for about 5 minutes. Wash mint, dab dry, chop leaves.
Mash the peas with a hand blender. Stir in mint. Season with salt and pepper. Keep warm.
In the meantime, heat frying oil in a deep fryer or a wide pot to approx. 180 °C (it is hot when small bubbles rise from a wooden skewer). Use a fork to pull fish pieces through the dough and fry them in portions in the hot oil for about 5 minutes, turning once.
Drain on kitchen paper. Dress everything. Go with potato wedges and tartar sauce.