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Heat oven to 200C/180C fan/gas 6. To make the pastry, tip the flour and butter into a food processor, and pulse until the mixture resembles fine breadcrumbs. Add the cheese and pulse to combine. Add 2-3 tbsp cold water and mix to a firm dough.
Tip the dough out onto a lightly floured surface and knead briefly. Roll out and line a 30 x 20cm rectangular tin (or a 26cm round flan tin) – no need to trim the pastry edges. Chill for 15 mins, then line with baking parchment, fill with baking beans and blind-bake for 10 mins. Remove the parchment and beans, and bake for a further 10 mins until biscuity brown. Trim off excess pastry using a small sharp knife.
Reduce oven to 180C/160C fan/gas 4. Melt the butter in a frying pan, add the spring onions and gently fry for a couple of mins until softened. Add the peas, season and cook gently, stirring occasionally, for a further 5 mins. Tip into the food processor with the milk and work to a purée.
Beat the eggs in a large bowl, then beat in the crème fraîche, nutmeg, half the cheese, the pea purée and seasoning. Pour into the pastry case and sprinkle with the remaining cheese. Bake for 25-35 mins until the filling is firm and lightly browned.