Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Peel onions and cut into strips. Heat a large pan without fat. Fry meat and onions for 5-8 minutes, turning them over.
Cover a baking tray with baking paper. Cut flat bread in half horizontally and place on top. First spread sauce, then meat on the bread. Rub the gouda directly over it. Crumble feta over it. Bake in a hot oven for about 15 minutes.
In the meantime, sort out the rocket, wash and drain. Take out the pizza. Put the salad on top. Add tzatziki.