For the pastry, place the flour in the fridge overnight. Mix the flour with the xanthan gum and salt. Cut the butter into small pieces and add to the flour. Now either use a food processor or your fingers and process or rub the mixture so some becomes breadcumb like but keep some streaks of butter in there.
Now add the egg and water a little at a time and quickly knead the dough to bring it together into a ball. If a bit too wet, add a little more flour.
Now flatten and shape the dough and roll into a fairly even rectangle, fold in half, rotate one turn clockwise and roll out one more time then fold in half and gently roll just to press together ( you still want a square shape). Wrap this and place in the fridge for 1 hour or overnight.
Preheat your oven to 180C. Roll the dough into an even 40cm circle between baking paper. Then remove the top sheet. You can use a knife to cut a circle out, just make sure its approx 40cm.
Gently spread the pastry with the mustard, leaving a 4cm boarder.
Toss the tomatoes with the garlic, oregano, olive oil and salt and pepper and pile thme over the mustard. Gently fold the boarder edges up and just over tomatoes, pleating the pastry as you do if desired. Brush this pastry edge and sides of the galette with the egg yolk and then place the tart, still on the baking paper, onto a baking tray and into the oven to cook for 20 - 25 minutes, until the pastry is golden and crisp.
Serve with a good crack of black pepper is desired. This tart can be made an hour or two in advance and be served at room temp.