Preheat the oven to 160ºC, fan 140ºC, gas 3. Heat the oil in a large flameproof casserole with a lid.
Add the onion and celery to the casserole and fry for a few minutes, then add the garlic, carrots and squash and fry for a further 2-3 minutes.
Add the tomatoes, stock and sugar. Give it all a good stir then cover and place in the oven for 20 minutes.
Remove from the oven and stir in the green beans. Return to the oven, uncovered, and cook for a further 20 minutes until the sauce has thickened; add the lamb for the final 10 minutes to warm through. Serve with the mashed potato.