Combine Pork and Bacon in a food processor and chop coarsely. Add pistachios and pulse a few times to roughly chop.
Transfer to bowl. Blend in salt and pepper. Mix in mushrooms, brandy, and Madeira. Blend well with a wooden spoon or hands, cover, and chill several hours or more in the refrigerator.
When ready to cook, divide puff pastry dough into 2 portions on a lightly floured work surface. 1 half should be slightly larger than the other. Roll both halves into ¼" thick rectangle.
Roll meat mixture into a 12" cylinder about 2" thick. Place cylinder in middle of smaller rolled dough rectangle.
Place remaining pastry sheet over the meat cylinder. Cut away excess pastry leaving just enough to enclose meat (reserve scraps for decorating). Pinch seams together and cut small slashes around sides of pastry. Place saucisson on a baking tray.
Preheat oven to 450°F (230°C). While oven heats, form excess pastry into a ball, then roll out to a ¼" sheet. Cut small crescents (or any shape you want, for that matter). Beat egg yolk with water and paint the whole cylinder with it. Arrange cut-outs over top and paint again.
Bake until golden brown, about 25 to 35 minutes. Let stand for 10 minutes before slicing into 1" pieces.
When Saucisson in in the oven, make sauce by boiling down beef stock by 25% in a small saucepan. Add Madeira and continue boiling down until everything is reduced to 1 cup. Turn off heat and whisk in butter slices 1 at a time.
Pour a little sauce (around 3 tbsp) on each small serving plate. Overlap 2 slices on top of the sauce. Serve to applause!