Place a pan over a medium heat with 1 tablespoon of olive oil, add the chopped onion and cook for 15 minutes until translucent, though not taking colour then set aside.
Bring a large pot of water to the boil.
Add the peas and boil for 4 minutes or until tender.
Drain the peas and add most of them to a food processor with the onion, garlic, mint, lemon zest and some salt.
Blitz the mixture, add 1 tablespoon of olive oil at a time until a thick purée is achieved.
Remove to a bowl and stir in the remaining peas.
Season with salt, pepper and a little more olive oil if necessary and keep warm.
If you have a deep-fat fryer, heat the oil to 190°C (375°F).
Otherwise pour 5cm sunflower oil into a medium saucepan and place over a medium-high heat. To check the temperature, a piece of bread should fry to golden in 30-40 seconds.
While the oil is heating, combine together the flours, baking powder, ½ a teaspoon of salt and a generous pinch of pepper.
Whisk in the cold sparkling water until there is a yoghurt consistency.
Whisk the egg white in a separate bowl until stiff peaks form, then gently fold into the batter.
Dip one of the cod fillets into the batter and coat completely.
Carefully place the fish in the saucepan of oil and fry until crisp and golden (about 5-7 minutes).
Only fry one or two fillets at a time. Don't overcrowd the pan.
Serve immediately with the mushy peas, a lemon wedge and oven chips if you like.