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 into 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, fry the bacon in a pan without fat until crispy.
Washing and cleaning mushrooms. Remove the bacon from the pan and fry the mushrooms in the bacon fat while turning for about 3 minutes. Wash the rosemary and shake dry. Peel and halve shallots and cut into rings.
Wash and halve the tomatoes. Mix sour cream and crème fraîche, season with salt and pepper. Quarter the yeast dough and knead again. Roll out a quarter oval (35 x 25 cm). Line a fat pan with baking paper.
Put the dough in. Spread a quarter of the sour cream on the dough. Spread a quarter of the mushrooms, tomatoes, shallots and rosemary evenly on top. Bake in the preheated oven (electric range: 250 °C/ convection oven: 225 °C/ gas: level 5) on the bottom shelf for 10-12 minutes.
In the meantime, process the remaining dough and topping in the same way. Bake the tarte flambée one after the other. Serve the finished tarte flambée with bacon warm.