Mix the flour and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Pour in the water and mix quickly to form a soft dough; do not knead. Flatten the dough, cover and leave to rest at room temperature for approx. 30 mins.
Heat the butter in a wide frying pan. Sauté the onions over a medium heat for approx. 15 mins., turning frequently, allow to cool slightly.
On a lightly floured surface, roll out the dough into a circle (approx. 32 cm in diameter), place in the prepared tin. Prick the base firmly with a fork.
Whisk the eggs with all the remaining ingredients, pour over the top of the onions.
Approx. 35 mins. on the bottom shelf of an oven preheated to 220°C. Remove from the oven, allow to cool slightly, remove from the tin.