Put the flour and salt in a bowl, add the margarine and mix until the dough looks like fine bread crumbs.
Add water to form a soft dough.
Allow to cool for 15-20 minutes.
Meanwhile, sauté the onions for 5 minutes.
Put the onions into a bowl, add the eggs, the milk and cheese and season.
Stretch the dough into the tart oven tray, 20 cm in diameter.
Pour the mixture and bake in the oven for 35-40 minutes.