Added 10/Sep/2014
source: 1, Tristan & Linde
For the béchamel:
For the mushroom mixture:
You can make the béchamel in advance, as for the rigatoni al forno above, as well as cooking the mushrooms, combining everything with the freshly cooked pasta just as it goes into the oven.
Soak the porcini in the boiling water in a small bowl. In a saucepan, melt the butter for the béchamel, and add the flour; cook gently to make a roux and then – off the heat – whisk in the milk. Turn the heat back on to medium, and stir the bechamel until it begins to thicken and come to the boil. Let it bubble for about 5 minutes to get rid of the floury taste. Take off the heat and season with salt, pepper and nutmeg.
Preheat the oven to 400°F. Heat a third of the butter and all of the oil for the mushrooms in a large wide pan. Drain the porcini, reserving the liquid, and chop before adding to the pan with half of the chopped parsley, the dried thyme and minced garlic. Stir for a couple of minutes then melt the remaining butter in the same pan and add the chopped mushrooms, stirring for about 5 minutes. The mushrooms will appear dry at first but will eventually start to give off some liquid.
Add the porcini soaking liquid, which the mushrooms will largely absorb, but keep stirring while adding the sherry and let it bubble away. Turn off the heat when you have a bronzed, syrupy stew. Stir the mushroom mixture into the bechamel and add half the Parmesan and the remaining half of the chopped parsley.
Put a big pan of water on for the pasta, and when it boils, salt it well. Cook the pasta until al dente, then drain and add to the mushroomy white sauce, stirring as best you can to get the pasta covered.
Turn into a large roasting pan, of approx. 12¾ x 16½ inches. Sprinkle over the remaining Parmesan and bake for 30 minutes or until the top begins to turn golden in places.
When the pan comes out of the oven, sprinkle with some more chopped parsley and some sprigs of fresh thyme.