1. Combine vinegar, shallots and tarragon in medium skillet; bring to a boil. Reduce heat to medium low and simmer until reduced to 1/3 cup.
2. Add 2 cups sherry and broth; bring to a boil. Reduce heat and simmer 25 to 30 minutes or until reduced to 1 cup.
3. Dissolve cornstarch in 1 tablespoon dry sherry. Add cornstarch mixture to skillet stirring constantly; cook and stir about 1 minute until sauce is slightly thickened. Whisk in mustard and honey. Remove from heat; whisk in butter. Serve warm.