Mix the yeast with 175 ml of lukewarm water and sugar. Knead with the flour, 1/2 teaspoon salt and oil until smooth. Cover and let the dough rise for about 45 minutes. Peel the onions and cut into fine rings, dice the bacon and grate the cheese.
Preheat the oven to 220 ° C, fan-assisted 200 ° C. Mix the sour cream with the egg yolk, season. Knead the dough again and cut in half. Roll out half very thinly on baking paper. Spread half of the sour cream, the diced bacon and the onion on top.
Sprinkle half of the cheese on the tarte flambée and bake individually for about 15 minutes. Prepare the rest in the same way; this is also possible at the same time with circulating air.
Rinse the rocket, drain well and clean.
Remove the tarte flambée, cover with rocket and serve.