Preheat oven and a baking tray to 200C. Lightly grease a 7cm deep, 20cm (base) springform pan.
Process flour and butter in a food processor until mixture resembles coarse breadcrumbs. Add egg yolk and water. Process until a soft dough forms, adding more water if required. Transfer to a floured surface. Knead gently until smooth. Wrap in greaseproof paper. Refrigerate for 30 minutes.
Roll dough between 2 sheets baking paper until 34cm in diameter. Ease into base and sides of pan. Trim to fit. Refrigerate for 30 minutes.
Line pastry with baking paper. Half-fill with uncooked beans or rice. Place on hot oven tray and bake blind for 10 minutes. Remove paper and beans and bake for 10 minutes. Set aside to cool.
Stir peas into chilled casserole. Spoon into pastry case. Brush pastry edges with egg. Top with puff pastry, pressing edges to seal. Trim excess pastry. Brush top with egg. Sprinkle with seeds. Bake for 40 to 45 minutes or until pastry is golden. Stand for 5 minutes. Cut into slices. Serve.
