Place leg of lamb in a large Dutch oven or cooktop safe roasting dish. Add salt, pepper and olive oil. Rub it in thoroughly. Brown the lamb on both sides over medium-high heat on the cooktop. Remove from heat.
Heat butter in a large saucepan. Add garlic cloves, bay leaves, leek, carrot and mushroom. Sauté on high heat for a minute. Add port wine to deglaze the pan. Remove from heat.
Pre-heat oven to 150°C.
Add the vegetable mix to the lamb. Gently pour the vegetable stock around the leg of lamb in the pan. Add rosemary. Cover the pan tightly with foil.
Place lamb in the oven and bake for 4 hours.
Remove lamb from the oven. It should be tender and falling apart. Use a fork to pull the meat off the bones and gently mash the vegetables into the sauce. Discard the bones, bay leaves and rosemary sprigs.
Heat the saucepan on medium. Add the shredded lamb and sauce to the pan. Add the passata, thyme and chilli flakes. Season with salt and pepper as required. Stir through the jam and reduce heat. Simmer until heated through.