1 large cauliflower
2 leeks, trimmed and cut into 2cm/戮in slices
200g/7oz button mushrooms, halved
2 tsp Dijon mustard
55g/2oz plain flour
salt and freshly ground black pepper
knob of butter
55g/2oz butter
115g/4oz mature cheddar, coarsely grated
1 free-range egg, beaten
450ml/戮 pint hot milk
55g/2oz Parmesan, coarsely grated (or alternative vegetarian hard cheese)
375g ready-rolled puff pastry
115g/4oz frozen petits pois