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Roll the pastry until just thicker than a £1 coin, and large enough to line a 23cm fluted tart tin (about 4.5cm deep) with some excess. Line the tin with the pastry, trim off the excess with a roll of the rolling pin, then prick the base all over with a fork. Chill for 15 mins. Heat oven to 200C/180C fan/gas 6. Line the pastry with a sheet of over-hanging foil and press it into the corners of the tin. Fill with baking beans, lift onto a baking sheet and bake for 25 mins until the pastry feels firm. Remove the foil and beans, and bake for another 5 mins until biscuity. Beat the egg yolk and brush it all around the inside of the case. Bake for another 5 mins until golden and lacquered. Turn the oven down to 180C/160C fan/gas 4.
Meanwhile, prepare the fish and filling. Put the fish on a plate, cover with cling film and microwave on High for about 4 mins until the flesh flakes easily. Alternatively, steam or gently poach the fish, then drain well. Gently transfer the fish to a colander and drain until the pastry is ready. Beat the 3 eggs, crème fraîche, chives and nutmeg with seasoning.
Flake the fish into large pieces, discarding any skin and bones, pat dry with kitchen paper and put into the pastry case. Tuck in the prawns here and there. Pour the filling over, scatter with the cheese, then bake for about 35 mins or until golden and set, with just a slight tremble in the middle of the tart.
For the salad, boil the peas for 1 min until just tender, then drain and cool in cold water. Drain well, put in a large bowl and top with the pea shoot salad and mint leaves; chill until needed. Mix the shallots with the sugar, vinegar, oil and mustard together to make a dressing. Season. Let the tart cool for a few mins before serving. When ready to serve, toss the salad dressing with the peas, pea shoots and mint.