Preheat the oven to 190C/375F/Gas 5.
Roll the puff pastry out to a thickness of ½cm/¼in. Chill in the refrigerator for at least 30 minutes Cut into a round shape with a diameter of about 20cm/8in and prick the whole surface a fork, evenly and completely all over. Place the pastry on a baking sheet, and bake for 10-15 minutes, until just nicely golden.
While the pastry is baking, sauté the vegetables in the butter for just a few minutes, until wilted, but not completely cooked. Season with salt and pepper.
Remove the pastry from the oven, and brush with the egg yolk wash. This helps to keep the vegetables sticking to the puff, but also, protects the pastry from absorbing any excess liquids from the vegetables, ensuring a crisp, not soggy base.
Spread the vegetables evenly over the pastry, sprinkle with the cheese and bake off for about 10 minutes, about 180C/350F/Gas 4. To serve, cut into wedges.
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