To make the polenta
Heat the vegetable stock in a medium saucepan and bring to a gentle simmer, add the bay leaf and thyme and then stir in the polenta. Turn the heat down a little and stir often, until it forms a porridge-like consistency, checking the polenta packaging for recommended cooking time. Once thickened, fish out the bay leaf and discard. Then stir in the yeast flakes and olive oil for extra gloss and flavour, and season to taste with salt and pepper.
To make the sautéed vegetables
While the polenta is cooking, cut the woody spine away from the cavolo nero. Blanch in boiling water for 2 minutes, strain and slice thinly. Heat the olive oil and garlic in a medium frying pan. Add the sliced mushrooms and thyme and stir-fry for 1–2 minutes. Add the balsamic vinegar and Worcestershire sauce, and fry until the liquid released from the mushrooms has nearly evaporated. Add the cavolo nero to the pan and fry for 1 minute. Finally, stir in the lemon zest.
Spoon the polenta onto the serving plates, and top with the sautéed vegetables. Grate over a little more lemon zest to finish.