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, clean, wash and cut leek into rings. Grate cheese roughly.
Wash the marjoram, shake dry, pluck leaves from the stalks and chop. Mix sour cream and crème fraiche with marjoram. Season with salt and pepper. Quarter the yeast dough and knead again. Roll out a quarter oval (35 x 25 cm).
Line the fat pan with baking paper. Put dough into it. Spread a quarter of the sour cream on the dough. Spread a quarter of the leek and cheese evenly on top. Bake on the lower shelf in the preheated oven (electric range: 250 °C/ convection oven: 225 °C/ gas: level 5) 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 Flammkuchen warm with Black Forest Ham.