Heat oven to 180C/160C fan/gas 4. Trim any excess fat from the lamb shoulder. Heat a large flameproof casserole dish and add the oil. Add the lamb shoulder and fry over a high heat to brown it on all sides. Remove from dish and pour off any excess fat.
Add the garlic cloves, bay leaves, juniper berries, red wine vinegar and white wine to the dish and set the shoulder on top. Cover tightly with a lid or foil and place in the oven to cook for 1 hr 30 mins, occasionally basting the lamb with the juices until the meat is tender. Remove the lid and cook the lamb uncovered for the final 10 mins, to reduce the cooking juices a little.
Remove the lamb from the oven, place on a board, cover and leave to rest for 15 mins. If the juices are not reduced in the dish, boil on a high heat until thickened. Season to taste. Serve the shoulder of lamb in slices with its cooking juices, and with the bay leaves, garlic and juniper berries on top.