To make the pastry, sieve the flour, salt and xanthan gum into a bowl. Place the butter and 130ml water into a small pan over a low heat and bring to the boil, then set aside to cool slightly. Beat the eggs, then stir into the flour mixture. Gradually add the warm butter until it comes together into a rough dough, then bring it together with your hands. Cover and leave to rest while you make the filling.
Preheat the oven to 180ºC/350ºF/gas 4. Line a large baking tray with greaseproof paper.
Trim and finely chop the leek. Melt the butter in a medium pan over a low heat, then add the leek and thyme leaves. Cook for 10 to 12 minutes, or until softened, then leave to cool. Pick and roughly chop the parsley leaves, then add to a bowl with the leftover veggies, mustard and a pinch of salt and pepper. Mash well, stir in the cooled leeks and grate in the cheese.
On a lightly floured surface, divide the pastry into 12 equal pieces, then roll each one out to roughly the thickness of a pound coin (if you find the pastry crumbles too much, roll it out between two large pieces of clingfilm). Using a saucer (roughly 12cm in diameter) as a guide, cut the pastry into circles.
Place roughly a spoonful of the mixture on one half of each pastry round, spreading it out slightly. Beat the egg, then brush it around the edges. Gently fold the pastry over the filling, pressing the edges to seal. Place on the prepared baking tray, brush with a little more egg, then pop in the hot oven for 35 to 40 minutes, or until golden and crisp, brushing with a little more egg halfway through. Serve hot!
TOP TIP If you’re short of time, you can buy ready-made gluten-free pastry in the freezer section of most supermarkets – just remember to fully defrost it before using.