Put flour and 1 large pinch of salt in a mixing bowl. Stir yeast with 1 teaspoon sugar until liquid and add to the flour.
Add approx. 175 ml lukewarm water and oil (see tip on the right) and knead to a smooth dough with the dough hooks of the mixer. It is best to knead for about 10 minutes, then it can be rolled out very thinly later.
Form the dough into a ball and cover with a kitchen towel. Leave to rise in a warm place for about 1 hour.
For the topping, peel onions and cut them into fine cubes. Mix crème fraîche and sour cream, season with salt and pepper.
Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer). Line baking tray with baking paper. Halve the dough. Knead one half again briefly and roll out oval and very thin on a floured work surface (see tip on the right). Place on the baking tray.
Spread half the crème-fraîche mixture on the dough with a tablespoon, leaving a rim of about 1 cm. Sprinkle with half of the ham and onions. Bake in a hot oven on the lowest shelf for 12-15 minutes. In the meantime prepare the 2nd tarte flambée in the same way. Take the finished tarte flambée out of the oven and cut it into pieces with a pizza cutter. Bake the 2nd portion.