Clean all the mushrooms very carefully and cut them up into even-sized pieces. Put them in a saucepan with all the other sauce ingredients. Stir everything together, cover and simmer for a generous hour, or until the mushrooms are cooked through and have re-absorbed their juices.
To make the polenta, pour the water into a large deep saucepan, preferably made of copper, and bring it to the boil with the salt. When the water boils, trickle in the polenta flour like fine rain, whisking constantly with a balloon whisk to prevent lumps from forming. When the mixture is smooth, cover with a lid and boil slowly, whisking very frequently. Cook for a total of 40 minutes, making sure it is boiling constantly.
Tip the cooked polenta out on to a wooden board, shape it into a cake using a spatula and let it set slightly. Cut it into slabs and serve 1 slab per person with the mushroom sauce.
© 1990 Valentina Harris. All rights reserved.