In a bowl, stir the baking powder into the flour and season with sea salt, quickly whisk in the cold beer or sparkling water until you have a thick paste. This needs to be done just before you cook the fish.
To make the minted mushy peas; bring a pan of salted water to the boil and add the frozen peas, cook gently for 3 minutes or until tender. Drain the peas and transfer to a blender, add the cream, butter, mint and season with sea salt and black pepper. Blend the ingredients together but retain a thick texture with small pieces of peas, place in a small saucepan.
Place the bowl near to where you will be cooking the fish and have a plate lined with kitchen paper to hand.
Heat the oil in a large pan or deep fat fryer to approximately 185°. Dip the fish into the batter and carefully lower into the oil, make sure it doesn’t stick. Cook the fish, one or two pieces at a time, repeat this process, don’t overcrowd the pan.
Cook the fish for 4-6 minutes each, until crisp and golden. Make sure you keep an eye on it, to ensure it doesn’t stick. Life from the oil and drain on kitchen paper.
Reheat the minted peas and serve with the fish, chunky chips, lemon wedges, tartare sauce and vinegar.