Place gluten free flour, salt and xanthan gum in a food processor. Pulse several times until well mixed.
Add cold butter cubes; continue to pulse or mix until it resembles coarse breadcrumbs.
Add in grated cheese, Dijon mustard and vinegar; pulse until ingredients are well mixed.
Add 5 tablespoons of cold water all at once and pulse. Keep on adding water, 1 tablespoon at a time, until a soft ball of dough forms which you can easily remove from the bowl.
Wrap dough in cling film and chill in the fridge for at least 1 hour.
Remove dough from fridge and let it sit at room temperature for 30 minutes. Divide dough into 3 or 4 equal parts. Work with one part at a time (wrapping the rest of the dough in cling film to prevent it from drying out).
Sprinkle gluten free flour over a rectangular piece of baking parchment. Put dough in centre of parchment, cover it with another sheet of parchment and roll dough into a rectangle 1/2 cm thick. Remove top sheet of parchment.
With a sharp knife, cut the dough into 2cm wide strips; rotate parchment 90 degrees and cut 2cm wide strips again in order to get square crackers. Gently separate the crackers and prick the dough generously with a fork.
Transfer baking parchment paper with crackers on it to a baking tray. Repeat same steps with remaining dough.
Bake in preheated oven for 10 minutes. Rotate baking pan 180 degrees and bake for another 12 minutes, or until golden and crisp.
Remove from oven and let them cool completely before storing. Store in tightly sealed container.