Preparation Spoon yolks from egg halves into small bowl; mash yolks to smooth paste. Add sour cream, mayonnaise, lemon juice and mustard; blend well. Mix in green onion, capers and tarragon. Season with salt and pepper. Spoon yolk mixture back into whites, mounding in center. (Can be made 8 hours ahead. Cover; refrigerate.)
Preparation Spoon yolks from egg halves into small bowl; mash yolks to smooth paste. Add sour cream, mayonnaise, lemon juice and mustard; blend well. Mix in green onion, capers and tarragon. Season with salt and pepper. Spoon yolk mixture back into whites, mounding in center. (Can be made 8 hours ahead. Cover; refrigerate.)