Mix the flour, salt, sugar and crumbled yeast in a bowl. Add water, mix and knead to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 1 1/2 hrs. until doubled in size.
Thinly slice the onions. Place the onions, vinegar and sugar in a pan and bring to the boil, simmer for approx. 20 mins., stirring occasionally until all the liquid has been absorbed.
Cut the cheese into thin slices. Divide the dough in half. On a lightly floured surface, roll out the dough into two rectangles (each approx. 20 x 30 cm). Place one half on a baking tray lined with baking paper. Top with the onions and cheese. Place the second piece of dough on top and press down firmly. Cover and leave to rise for a further 30 mins. Using a knife, mark out 16 pieces (approx. 1 cm deep). Place the herbs in the incisions, drizzle with oil.
Bake for approx. 25 mins. in the centre of an oven preheated to 220°C. Allow to cool a little on a rack.
Serve with green salad.