Mix the flour and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Combine the yeast and water, pour into the flour 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.
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.
Heat the clarified butter in a frying pan, add the diced bacon, fry for approx. 3 mins., reduce the heat. Add the onions, sauté over a medium heat for approx. 15 mins., turning frequently, allow to cool slightly, spread on top of the dough.
Whisk the milk together 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.