Combine first 3 ingredients in a small bowl; stir to dissolve sugar.
Heat oil in a large nonstick skillet over medium-high heat; add mushrooms, and sauté 4 minutes or until tender. Add 1/3 cup sherry mixture to mushrooms. Reduce heat, and simmer 1 to 2 minutes or until liquid almost evaporates. Spoon mushroom mixture into a bowl; set aside.
Heat pan over medium-high heat; add fillets. Cook 3 to 4 minutes on each side or until browned on all sides. Add mushrooms and remaining sherry mixture to pan; cook 2 minutes. Transfer fillets to a serving platter, and top with sauce and mushrooms.