In a heavy saucepan over high heat bring water to a boil with salt and pepper. Slowly pour in polenta stirring constantly with a wooden spoon. Reduce heat to low and cook 20 minutes; stirring frequently as the mixture thickens.
Using a wet spatula, spoon the polenta mixture 1cm thick onto a sheet of greaseproof paper. Set aside to cool.
Preheat oven grill. Lightly grease a baking tray.
Heat oil in a large frypan over high heat. Cook mushrooms, onion, garlic and herbs in the hot oil until mushrooms are tender.
Cut cooled polenta into squares and place on prepared baking tray. Grill until browned on both sides. Serve with mushroom sauce.