Crumble the yeast and mix with sugar until the yeast becomes liquid. (This saves the pre-dough.).
Put flour, 1/2 teaspoon salt, oil and 300 ml lukewarm water in a bowl. Add mixed yeast and knead immediately to a smooth dough. Cover the dough and let it rise in a warm place for about 30 minutes.
In the meantime, peel and halve the onions and cut them into rings. Cut bacon into strips. Mix sour cream and crème fraîche, season with salt and pepper.
Quarter the yeast dough and knead again. Roll out one quarter oval (35x 25 cm). Line a fat pan with baking paper. Put dough into it. Spread a quarter of the sour cream on the dough. Spread a quarter of the bacon and onions evenly on top.
Bake in the preheated oven (electric cooker: 250 °C/ convection oven: 225 °C/ gas: level 5) for 10-12 minutes in the lower rack of the oven. In the meantime, process the remaining dough and topping in the same way and bake.
Serve the finished tarte flambée warm.