Preheat oven to 400 Degrees.
Soak the Porcini Mushrooms in 1 Cup warm water for about 15-20 minutes. Drain and set aside
Place Pistachios into a food processor and process until finely ground. Set aside.
In a large saute pan heat the olive oil and saute the Shallots until crisp at the edges. Add the garlic and saute until golden brown. Add the sausage and saute for 5-7 minutes until everything is well browned. Add the Porcini and the Wine and reduce. Remove from the heat and allow the mixture to cool.
In a small sauce pan add the flour to the Butter once it's melted until it begins to brown. Add the milk whisking until the mixture thickens. Add Salt to taste and Nutmeg. Set aside.
Cook the Paccheri half way through in salted boiling water. Drain the pasta and run under cold water. Add a few drops of oil to prevent sticking. Set aside.
In a large mixing bowl combine the cooled Sausage mixture, Ricotta, Pistachios and Parmigiano. Mix well and transfer into a disposable pastry bag and snip off the tip so the opening is about 3/4 of an inch wide. (a ziplock bag will also work, just fill and snip off a corner)
In a round pan about 11 inches in diameter spread a thin layer of Bechamel on the bottom. Arrange the Paccheri standing upright in the pan squeezing them together as best you can without breaking any. If there are a few left over try to arrange them in another smaller pan or discard them.
Now fill each of the Paccheri to the top with the pastry bag. Top with the remaining Bechamel and Sprinkle extra Parmigiano on top. Bake for about 30 minutes or until the top is golden brown. Serve immediately.