To make the cod, pat the fish dry with a clean tea towel or kitchen paper and season with salt and pepper.
Blitz the pistachio nuts to crumbs in a food processor and tip into a large bowl. Add the panko breadcrumbs and matcha powder and stir well. Whisk the egg in a second bowl. Put the flour into a third bowl. Set aside.
To make the mushy peas, cook the peas in a saucepan of boiling water for 5–7 minutes, or until very soft. Drain and return to the pan. Add the butter, mint, oil, salt, lemon juice and wasabi paste. Using a potato masher, mash into a thick, slightly lumpy purée. Season and set aside.
To make the chips, rinse the sweet potatoes in cold water and pat dry on a clean tea towel. Mix the cornflour with ½ teaspoon of salt and 75ml/2½fl oz of water in a large bowl to make a thin paste. Add the sweet potatoes and toss until well coated.
Heat the sunflower oil in a deep-fat fryer or deep saucepan to 180C. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Add half the chips using tongs, a chip at a time so they don’t stick together, and fry for 3 minutes, or until crisp. Remove with a slotted spoon and drain on kitchen paper. Repeat with the remaining chips. Set aside.
To cook the fish, dust one of the fish fillets all over in the flour, dip into the beaten egg and then into the crumb mixture, turning until evenly coated. Repeat with the remaining fish pieces.
Use a spatula to carefully lower two of the fish fillets into the fryer, then cook for 3–4 minutes, or until golden-brown. Carefully turn over and fry on the other side for a further 3–4 minutes, or until cooked through. The time can vary according to the thickness of each fillet. Drain on kitchen paper then transfer to a low oven to keep warm while you cook the remaining fish fillets.
To make the tempura peas, put the cornflour, flour, egg and iced water in a bowl and whisk until smooth. Add the peas to the batter, and toss until well coated. Drop half the peas, one at a time, gently into the hot oil using tongs and cook for 1½–2 minutes, or until the batter is crisp and the peas are tender. Remove with a slotted spoon and drain on kitchen paper. Repeat with the remaining peas.
Return the chips in two batches to the hot oil, very carefully and cook for 1–2 minutes more, or until nicely browned and crisp. Drain on kitchen paper, sprinkle with nori flakes or a little sea salt and serve with the cooked fish, mushy peas, tempura peas and lemon wedges for squeezing.