Melt butter in a large skillet over medium-high heat; add mushrooms and shallots. Sauté 6 to 7 minutes or until mushrooms release moisture. Sprinkle with salt and pepper; stir well. Sprinkle with flour, stirring to coat mushrooms; cook 1 minute. Add sherry, broth, and tarragon; simmer, uncovered, 10 to 15 minutes or until liquid is absorbed, stirring occasionally. Remove from heat; stir in sour cream.
Spoon mixture into a serving bowl. Garnish, if desired. Spread on melba toast rounds or crostini.