1. Mix the cold water with the apple cider vinegar and put this into the freezer until you need it.
2. Sieve the gluten-free flour with the xanthan gum and the salt in a bowl.
3. Add the cubes of butter and knead this with your fingers until you have fine crumbles.
4. Add the cold water-vinegar mixture. Add a tablespoon at a time and mix the dough with a for until it starts to stick together. You need about 6-8 tablespoons.
5. Form the dough into a disk and wrap it in cling film and leave to rest in the fridge for at least 1 hour.
6. Pre-heat the oven to 200 degrees Celsius (390 F).
7. Roll out the dough until it is about 2-3 mm thin and larger than the pie tin in diameter, about 28-30 cm (11-12 inches).
8. Carefully lift up the pie crust and lay it over the baking tin. Carefully press it into the tin. Remove any pastry that is sticking over the edge.
9. Refrigerate the pie crust for another 15 minutes.
10. Prick the bottom of the crust with a fork and bake for 10 minutes.
11. Lower the oven temperature to 180 degrees Celsius (350 F)

The filling

1. Heat some oil in a pan and fry the spinach until softened.
2. Cut the spring onion into rings and add to the spinach. Season with salt and pepper and turn off the heat.
3. Cut the salmon into pieces.
4. Whisk the eggs together with the crème fraîche. Stir in the spinach and smoked salmon. Season with salt and pepper.
5. Divide the mixture over the crust and sprinkle the grated cheese on top.
6. Bake the quiche in the oven for 40-50 minutes. Keep an eye on the quiche while baking. If the filling gets too dark, you can cover the quiche with aluminium foil.