Bring the water to a boil in a heavy-based pot and add the salt. As soon as the water boils, add the polenta in a gentle stream, whisking vigorously as you pour it in. Reduce the heat so that the mixture is simmering, and stir continuously for the first 30 seconds to prevent any lumps from forming. The more you stir throughout, the better the texture will be. Cook for 40 minutes, stirring every few minutes. When done, add the butter and Parmesan and season with freshly milled black pepper to taste. Spoon the polenta into a non-stick loaf tin and keep to one side to cool and set.

Place all the ingredients for the paste in a dish and blend to a smooth paste using a hand-held blender. Keep to one side until you are ready to use it.

Heat the olive oil in a pan and fry the mushrooms until brown. Place on a paper towel for the oil to drain.

Preheat the oven on the highest grill setting and place the oven rack directly underneath the grill (as close as possible).

Turn the polenta loaf upside down and cut into slices about 1.5 cm thick. Place the slices onto an oven tray, spread with the paste and place under the grill until they begin to brown.

Top each slice with mushrooms and Parmesan shavings, and serve hot.

Tip

You can prepare the polenta 24 hours in advance.