Preheat the oven to 200°C/180°C Fan/Gas Mark 6. Toss the prepared butternut squash and sweet potatoes with the red onion and olive oil. Place on a large baking tray, season well and roast for 25 minutes until tender. Then set aside to cool. Roll out a third of the pastry to a 20cm x 30cm rectangle and place on a lined baking sheet. Pierce all over brush with egg and bake for 15 minutes until golden and risen.
Roll out the remaining pastry to a rectangle slightly larger than the base then using a sharp knife slash diagonally. Spoon the roasted vegetables over the cooked base leaving a small border. Crumble over the Boursin. Brush the border with egg then carefully lift the slashed pastry and place over the vegetables. Press down the edges to seal then brush the pastry with egg. Bake for 20 minutes until the pastry is golden.
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