Saute shallots in butter in a large skillet over medium-high heat 2 minutes; add mushrooms and garlic. Cook 5 minutes or until liquid evaporates. Add vermouth; cook until liquid evaporates. Remove from heat; stir in thyme, 1/4 teaspoon salt, and 1/4 teaspoon pepper.
Sprinkle fillets with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook fillets in a nonstick skillet over high heat 2 minutes on each side.
Roll each pastry shell into a 7-inch circle. Spread about 1/4 cup mushroom mixture in center of each circle. Top with fillets. Wrap pastry over each fillet, tucking edges under fillet and pinching to seal. Place, seam side down, on a rack in a broiler pan.
Bake at 450° for 15 minutes or until golden. Serve with Cucumber Sauce. Garnish, if desired.
Delicious and very elegant presentation to dress up salmon! The flavors were great wonderfully rich. The only change I would make is to use puff pastry sheets rather than the shells, which were difficult to roll out. I would just trim the full sheets down to size in future rather than wrestle with the rolling pin. The cucumber sauce was delicious too. Highly recommended!