In a medium lidded casserole dish, heat the oil over a medium-high heat on the hob. Brown the chicken for 5-8 minutes, turning so it's golden brown all over. Remove from the pan and set aside.
Reduce the heat to medium, and add the bacon to the pan. Fry until starting to crisp, and then add the onion. Fry for another couple of minutes until the onion is starting to soften and then add the garlic, rosemary and bay leaf. Cook for a further minute before returning the chicken to the pan.
Add the passata and stock to the casserole. Bring to a gentle simmer over a low heat and then cover. Cook for 1 hour, removing the lid for the last 15 minutes of cooking time. At the end, stir through the olives to warm through. Season with freshly ground black pepper.
While the casserole is cooking, make the mashed potato. Put the potatoes in a pan and cover with cold water. Bring to the boil and simmer for around 20 minutes, or until really tender. Remove from the heat, drain and leave the potatoes in the colander for a couple of minutes until most of the steam evaporates. Return to the pan and add the butter. Mash until smooth and season with freshly ground black pepper.
Serve the chicken with the mash.