Mix the flours in a large bowl and make a well in the centre. Crumble in the yeast and add the sugar and 4 teaspoons of the water. Mix with a little of the surrounding flour to a spongy texture. Cover the bowl with a cloth and leave to ‘prove’ in a warm place for about 30 minutes, until the ‘sponge’ has doubled in size.
Add the caraway seeds, 4 teaspoons of the crème fraiche, salt and the remaining water. Mix with a spoon, bringing in the flour from the sides, to form a smooth dough. Cover the bowl with a cloth and leave in a warm place for 30 minutes.
Meanwhile, sweat the onions in the butter over low heat without colouring. Allow to cool slightly and then mix in the beaten egg and half of the remaining crème fraiche. Season the taste with salt and freshly ground black pepper.
Roll out the dough on a lightly floured surface to a thickness of 5mm. Grease a baking sheet and line with the dough. Brush lightly with a little water and then spread the onion mixture over the top. Cover lightly with the remaining crème fraiche and arrange the bacon rashers over the top.
Bake in a preheated oven at 180?C for about 40-50 minutes, until the base is crisp and brown and the topping is golden. Serve hot cut into slices.