Preheat oven to moderate, 180°C.
Heat half the oil in a flameproof casserole dish on high.
Brown lamb chops in batches, for 2-3 minutes each side. Transfer chops to a plate.
Heat remaining oil in same pan on high. Saute carrot, celery, onion and garlic for 3-4 minutes, until onion is tender.
Sprinkle flour over vegetables. Cook for 1 minute.
Blend in stock, tomatoes, tomato paste and rosemary. Bring to the boil, stirring constantly.
Reduce heat to low and simmer for 5 minutes. Season to taste.
Return chops to dish, turning to cover with sauce.
Place potato slices evenly on top. Dot with butter.
Bake, covered, for 2 hours.
Remove lid and cook further 30 minutes, until potatoes are golden and lamb is tender.
