Turn the oven to 180 degrees Celsius. For the base, half one pastry sheet and place the halves over two adjacent sides of the pastry sheet so they're slightly overlapping. Press the edges firmly to seal. It's best to use a palette knife or a fork to press the pastry together.
Now carefully line your tart tin with the pastry and trim the excess. If you get any cracks or tears in your pastry, just press it back together or patch it up with some of the excess pastry. To blind bake the pastry case, line it with baking paper and fill with pastry weights or rice. Place the tart tin on a baking tray and bake for 10 minutes. Carefully lift off the paper with the weights or rice and pop it back in the oven for another 10 minutes or until it's a light golden colour.
While the pastry is cooking, cook the veggies for the filling. Spray a non-stick frying pan with oil. Heat over medium heat. Add leek, garlic and mushrooms. Stir for five minutes until the vegetables start to soften. Add the spinach and keep cooking for two minutes until the spinach wilts. Turn off the heat and leave the mixture to cool slightly.
For the egg mixture, whisk eggs and milk in a jug until well combined. Arrange the spinach mixture in the pastry case and crumble in some fetta. Pour over the egg mixture. Bake for 25 to 30 minutes or until the filling is just set. Leave the tart for 10 minutes to cool before cutting into wedges. This gluten-free spinach and mushroom tart is great for lunch or dinner. For another gluten-free recipe, check out our gluten-free sweet potato and fetta rolls here.