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On a table, quickly combine the pastry ingredients, mixing until firm, then leave in the fridge.
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In a frying pan, fry the chopped onions, oil and sausage until lightly browned.
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Leave to cool.
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Preheat the oven to 180 C / 356 F.
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Roll the pastry to form a square 10 x 10 inches.
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Line a baking dish with baking paper, and lay the pastry on it.
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Break the egg and add it together with the Parmesan to the cooled pan mixture.Mix it all together. Spread the mixture over the pastry, combining with the butternut squash slices, leave a 1.5 inch border around the edges.
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Overlap and fold the pastry, pressing the overlapping edges together.
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Gently spread butter over the top of the filling.
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Egg wash the folded pastry.
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Bake for 20 minutes.
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Garnish with rosemary and serve.