Chop the lamb leg steaks into small chunks about 2cm across. Warm 2 tsp oil in a pan over a medium heat. Add the lamb. Fry for 8-10 mins, turning once or twice till the lamb is browned all over.
While the lamb cooks, peel and finely chop the onion. Finely slice the celery stick. Trim and peel the carrot, then finely chop. Peel and crush the garlic.
Add the onion, celery and carrot to the lamb. Season well with salt and pepper. Cook and stir for 5 mins till the veg are soft and glossy looking.
Add the garlic to the pan with most of the thyme sprigs. Cook and stir for 1 min.
Pour in 500ml boiling water. Cover. Bring to the boil, then turn the heat down and simmer for 45 mins till the lamb is tender.
Shred the cavolo nero. Drain and rinse the butter beans. Fish the thyme sprigs out of the lamb stew. Add in the cavolo nero and beans. Simmer, uncovered, for 5 mins.
Taste the stew and adjust the seasoning if you think it needs more salt and pepper. Ladle into warm bowls. Scatter over the last of the thyme leaves to serve.