Preheat the oven to 190°C/375°F/gas 5. Peel the onion and cut into eighths, trim, wash and slice the leek, peel and slice the garlic. Place a large ovenproof casserole pan on a medium heat. Finely slice the pancetta, pick and finely chop the rosemary leaves, then place both in the pan with 1 tablespoon of oil and the bay leaves. Stir regularly for 2 minutes, then add the garlic, followed by the onion and leek. Cook for 10 minutes, stirring regularly.