In a large skillet, over high heat, saute tournedos in 3 tablespoons butter for 30 seconds on each side. Set aside to cool completely.
In the same skillet, with the remaining 3 tablespoons of butter, cook shallots and mushrooms until all liquid evaporates. Add cream, salt and pepper. Reduce over moderate heat to a thick puree. Cool.
Preheat oven to 450 degrees F.
Divide pastry into 6 equal portions. On a lightly floured board, roll out each portion to a rectangle approximately 6 by 12 inches and 1/4 inch thick. Place each tournedo on one side of a pastry rectangle, leaving enough room to fold over.
Top each tournedos with mushroom puree. Brush edges of the pastry with egg wash and then fold it over and shape it to the contours of the meat. Cut away excess pastry and decorate as desired.
Glaze the tops of the pastry with egg wash.
Bake the tournedos 15 to 20 minutes, or until the pastry is golden brown.
Arrange each tournedo on a heated plate and garnish with watercress. Nap the sauce Madeira around the tournedos.
In a saucepan, bring Madeira, shallots, and thyme to a boil. Reduce to a simmer and cook until only 1/3 cup remains. Add the stock; continue to reduce, over medium heat, until slightly thickened. Whisk in the butter. Season, to taste, with salt and pepper.