Crumble the yeast and mix with sugar until the yeast becomes liquid. (This saves the pre-dough.) Put flour, 1/4 teaspoon salt, oil and 175 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
Season sour cream with a little salt and pepper. Peel red onions and cut them into rings. Clean and wash spring onions and cut them into slanted rings. Chop nuts coarsely
Line baking tray with baking paper. Halve the dough. Put 1 half on a baking tray and roll out very thin oval. Spread half the sour cream on the bottom. Cover with half of the red onions and spring onions. Sprinkle half of the nuts on top. Bake in the preheated oven on the lower shelf (electric oven: 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
Take out the tarte flambée. Bake the 2nd tarte flambée in the same way. Cover the tarte flambée with ham, sprinkle with pepper
Waiting time approx. 30 minutes