Pre-heat oven to 200°C fan. Unroll the pastry and cut out 2 discs. Make one slightly larger than the other.
Cook the leeks in the oil with the garlic, thyme and season. Add the diced chestnuts.
Cook the whole cauliflower in salted water until tender, then cool.
For the cheese sauce, melt the butter in a pan and then add the flour. Add the milk a little at a time and whisk until smooth with no lumps. Add the mustard, tabasco, 100g Cheddar cheese, 50g Parmesan and the truffle oil. Leave to cool a little.
Pour the sauce over the cold cauliflower until it has an even thick layer and sprinkle with the remaining cheese.
To assemble the pithivier, place the smaller puff pastry disk onto a parchment lined baking tray. Add the leeks mixture to the puff pastry, leave a 2cm edge. Sit the cheesy cooked cauliflower on top of the leeks then brush the edge of the pastry with a little bit of the beaten egg.
Place the larger puff pastry disc on top and seal the edges. Be careful to have a nice seal with no gaps. Brush the remaining beaten egg on top and cut a few holes in the centre.
Crimp the edges tight with a floured fork and bake for 25-30 minutes until golden and serve with green vegetables