Place any leftover lamb roast bones on a roasting tray and scatter the unpeeled, quartered onion, halved carrot, celery stick and garlic. Drizzle with the olive oil and roast in the centre of the oven for 45 minutes.
Transfer the roasted bones and vegetables to a large stockpot. Cover with water and bring to the boil. Reduce to a simmer and cover, leaving the lid slightly ajar, for four hours or so.
Strain off the bone broth and refrigerate or freeze until needed.
To prepare the soup, heat the butter and sunflower oil in a soup pan.
Saute the onion, stirring occasionally while you dice and add the remaining vegetables.
Add the diced lamb, pour over the lamb bone broth and bring to a simmer.
Simmer gently for 20 minutes, until the vegetables are tender.
Carefully transfer a few ladlefuls of the chunky soup to a blender and blend until smooth. Alternatively, use a hand blender and pulse for a few seconds to thicken the soup somewhat. Season well to taste.
Serve warm with buttered crusty bread or buttered bannocks.
You could add a handful or two of chopped kale for the final five minutes of cooking, if desired.