Mix the flour and salt. Add the butter and rub together using your hands to form an even, crumbly mixture. Pour in the milk, mix to form a soft dough. On a lightly floured surface, roll out the dough (approx. 32 cm in diameter), transfer to the prepared tin, prick the base with a fork.
Heat the oil in a pan, briefly sauté the onion, add the spinach, cover and warm for approx. 15 mins. Remove the pan from the heat, allow to cool slightly. Whisk together the cream, eggs and cheese, season, mix with the spinach. Spread the spinach mixture over the pastry base.
Approx. 30 mins. in the lower half of an oven preheated to 220°C.