Preheat the oven to 200c/390f.
Allow the shortcrust pastry to defrost on the bench for five minutes until it is soft enough to be shaped with your hands into the tin. You may need just over 1 sheet to cover all the edges, tearing bits off to cover the whole tin and pressing around the edges to remove any holes or gaps.
Place the pastry into the oven to blind bake for 10-15 minutes until very lightly golden (this will avoid having a soggy crust).
Add all of the dry ingredients and the water to a large bowl and mix together with a whisk to remove any lumps.
Add the vegetables & cheese and stir to combine.
Remove the pastry base from the oven, and then pour in the quiche mix, gently spreading with a spoon to evenly spread it.
Place back in the oven to cook for a further 40-45 minutes until golden on top and a skewer comes out clean (if yours isn't fully cooked at 45 minutes, keep checking every 2 minutes, and keep in mind that it will firm up as it starts to cool down).
Once cooked, remove from the oven and allow to cool in the tin for 5 minutes before removing and serving.