Peel and finely dice the onion. Cut the dough in half crosswise. Roll out both halves one after the other on a floured work surface. Place on 2 baking trays lined with baking paper. Spread each half with crème fraîche.
Spread onion and diced ham on top. Bake the trays one after the other in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas mark 4) for about 5 minutes. In the meantime, wash and clean the rocket and shake it dry. Take out the tarte flambée, cut into pieces and cover with rocket