Heat oil in a heavy-based saucepan. Add the onions and cook over low heat for 40 minutes until completely soft and golden. Season to taste with salt and pepper. Stir in the chopped herbs.
Spoon the onions on top of the flat bread and spread evenly. Scatter over the black olives. Place the flat bread on a baking sheet and bake for 15 minutes until crisp and golden. Scatter over the rosemary sprigs and oregano leaves.Serve immediately.