Peel and finely chop the onion. Trim, peel and finely chop the carrot. Trim the celery ends and finely slice. Peel and grate or crush the garlic.
Pour 750 ml boiling water into a high-sided pan. Bring to the boil. Trickle in the polenta, whisking as you go. Once it’s all in, turn the heat down low and cover. Cook for 40 mins, stirring now and then, till the polenta is thick, creamy and pulls away from the side of the pan as you stir it.
While the polenta cooks, warm 2 tsp oil in a deep frying pan or wok. Add the beef stir-fry strips. Fry over a medium heat for 5 mins, stirring now and then, till browned all over.
When the beef is brown, add the chopped veg. Season well. Fry for 5 mins, stirring now and then, till the veg start to look soft. Add the garlic and most of the thyme sprigs.
Pour in 450 ml boiling water. Cover. Bring to the boil, then turn down the heat. Simmer for 30 mins till the beef is tender. Slice the thick cores from the cavolo nero and shred the leaves.
Add the cavolo nero to the beef stew. Simmer, without the lid on, for 5 mins till the cavolo nero is cooked through. While the cavolo nero cooks, chop the butter into cubes.
Stir the butter into the polenta and season well. Spoon into warm bowls. Top with the beef stew and serve straight away, garnished with the last of the thyme leaves.