Pour 750 ml boiling water into a high-sided pan over a medium heat. 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 30 mins, stirring now and then till the polenta is thick, creamy and pulls away from the side of the pan as you stir it.
Trim the ends off the celery and finely slice it. Finely slice the mushrooms. Warm a deep frying pan or wok over a medium heat for 1 min. Add 1 tbsp oil, the celery and mushrooms. Season. Fry and stir for 10 mins till the celery has softened and the mushrooms have picked up a little colour.
While the mushrooms and celery cook, roughly chop all the chestnuts. Peel and grate or crush the garlic. Strip the leaves off the rosemary sprigs. Roughly chop them.
Stir the garlic and most of the rosemary leaves into the frying pan or wok. Cook and stir for 1 min, then add the chestnuts.
Crumble the stock cube into the frying pan or wok. Pour in 200ml boiling water. Cover and simmer for 10 mins till the liquid has reduced by half and the celery and mushrooms are tender. Stir now and then.
Chop the butter into the polenta and stir till it melts. Taste and add a little salt and pepper if you think it needs it.
Spoon the polenta into warm bowls or plates. Top with the chestnut and mushroom sauce and scatter
over the last of the rosemary to garnish.