Have fun preparing this mushroom dish!
(*) For a casserole dish of 20x20cm, enough for four to six people.
In a heavy saucepan large enough (*) bring water and salt to a boil at high temperature. Pour polenta semolina leisurely into the boiling water, which must continue to bubble. Stir continuously to prevent lumps from forming. Reduce the heat and simmer the polenta, stirring frequently, for about twenty to thirty minutes, until cooked and thickened (perhaps adding a tiny bit more water) and the spoon remains upright in the center of the pot. Spoon onto a large buttered baking sheet form and spread with a metal spatula or possibly kitchen knife to form a thin layer about 20cm by 40cm (**). Place the polenta in the refrigerator for about two hours.
For the béchamel: In a heavy saucepan large enough, melt the butter (1) over moderate heat and stir in the flour.
Remove the saucepan from the heat and, stirring constantly with a whisk, add the milk and whipping cream, whisking until the flour is partially dissolved. Then place the saucepan over a hot flame one more time and heat, stirring constantly with the whisk, until the sauce comes to a boil and is thick and smooth. Reduce the heat and simmer for a few min, stirring constantly, until the flour no longer tastes raw. Take the saucepan off the heat