Before you start, preheat your oven to 200°C/400°F/Gas Mark 6
To make the pastry, place the Mix and salt in a bowl and rub in the fats until the mixture resembles coarse breadcrumbs.
Stir in the beaten egg and sufficient water to form a soft but not sticky dough.
Knead well on a surface lightly dusted with Mix. Roll out the pastry into a rectangle to fit a large swiss roll tin, pushing the edges up the sides of the tin.
Place the potatoes in a pan of cold water, bring to the boil and cook for 5 minutes until almost cooked through. Drain and set aside to cool.
Heat the oil in a large pan; add the onions and cook for a few minutes until softened. Stir in the garlic and half the thyme and cook for a couple of minutes.
Spread the mustard evenly over the pastry base, layer the onions on top and then scatter the potatoes and Brie over the top.
Beat the egg with the crème fraiche, season well, and pour over the tart.
Sprinkle with the remaining thyme and bake in a pre-heated oven for 20-30 minutes.
Allow to cool in the tin for a few minutes before carefully transferring to a chopping board or serving plate.