Servings:  Serves eight to ten For the sauce: 1-1/4 ounces  (2-1/2 tablespoons) unsalted butter 1 large  shallot, finely chopped (about 1/4 cup) 1   750-milliliter bottle tawny port (about 3-1/4 cups) 2 sprigs  fresh thyme 2-1/2 cups  low-salt chicken broth 1 ounce  dried porcini mushrooms 2 teaspoons  all-purpose flour 1 teaspoon  good-quality balsamic vinegar; more to [...]