Preheat the oven to 200°C, fan 180°C, gas 6. Line a large baking tray with baking parchment.
Place the chips on the tray and toss with the olive oil. Season with salt and cook in the oven for 40 minutes, turning once, until crisp and golden.
Meanwhile, sift the flour and salt into a large bowl. Whisk in the lager until you have a smooth, lump-free batter with the consistency of double cream – it should be thick enough to coat the back of a wooden spoon.
Pour enough sunflower oil into a large, deep and heavy pan to come halfway up the sides. Heat over a medium heat until a cube of bread sizzles and crisps up within 30 seconds of being dropped into the oil.
Season the fish fillets with salt and freshly ground black pepper and dust lightly with flour. Dip 2 fillets into the batter, shaking off any excess. Using tongs, carefully lower the fillets into the hot oil and cook for 2-3 minutes on each side, until crisp, golden and cooked through. Drain on kitchen paper and keep warm. Repeat with the remaining 2 fillets.
Meanwhile, place the peas in a pan of boiling water and simmer for 2-3 minutes over a medium heat. Drain, reserving 2 tablespoons of the cooking water. Stir the mint, crème fraîche and reserved water into the peas. Crush with a fork and season to taste with freshly ground black pepper. Serve with the fish and chips, with lemon wedges on the side.