In a small saucepan, bring water and porcini mushrooms to a boil. Remove from the heat; let stand for 30 minutes.
Meanwhile, make a lengthwise slit down the center of the pork to within 1/2 in. of bottom. Open meat so it lies flat; cover with plastic wrap. Flatten to 3/8-in. thickness. Remove plastic wrap; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Set aside.
Crumble the sausage into a large skillet; add 2 cups shiitake mushrooms and onion. Cook over medium heat until meat is no longer pink. Remove from the heat; stir in the bread crumbs, 3 teaspoons parsley, rosemary and sage.
Drain porcini mushrooms, reserving liquid. Chop mushrooms; stir into sausage mixture along with 3 tablespoons soaking liquid. Spoon stuffing down center of pork; bring long sides over stuffing to close. Tie at 2-in. intervals with kitchen string; secure ends with toothpicks.
Place on a rack in a shallow roasting pan. Bake, uncovered, at 425° for 30-35 minutes or until a thermometer reads 160°. Let stand for 5 minutes before slicing.
For sauce, in a small saucepan, saute remaining shiitake mushrooms in butter until tender. Sprinkle with flour; stir until blended. Gradually add broth and 1/4 cup mushroom soaking liquid. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in the remaining salt, pepper and parsley. Serve with pork.