Preheat the oven to gas mark 6. Unroll the pastry onto a baking tray. Score a 3 cm border into the pastry (don't cut all of the way through!) and then lightly score the inner section with a criss-cross pattern. Beat the egg in a bowl and eggwash the whole thing.
2.
Bake for around 17 minutes, until golden. Once done, remove from the oven and leave to cool slightly. Using a sharp knife lightly score around the border, cutting through the top through layers of the pastry only and then use a fish slice to carefully lift up and remove the inner section (like a lid) and put to one side.
3.
Meanwhile, peel and finely chop the onion. Put into a medium pan with a splash of oil and cook for around 10 minutes, stirring occasionally. Peel the potatoes and cut into 1cm cubes and then stir into the pan along with the peas, butter and flour. Finely shred and add the lettuce.
4.
Cook for another 10 minutes and then crumble in the stock cube and pour in 150 ml boiling water. Bring to the boil and then cover and simmer for 5-10 minutes until the potato is cooked.
5.
Stir in the mustard and mint sauce and then mix in the cottage cheese and pour into the pastry case.
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