Combine the first 4 ingredients in a small heavy saucepan over medium-high heat; bring to a boil. Cook until the liquid is reduced to 2 tablespoons (about 9 minutes). Remove from the heat, and strain through a fine sieve over a measuring cup, pressing the mixture to release liquid. Discard the solids. Return the liquid to the pan.
Add butter, 1 piece at a time, stirring with a whisk until the butter is incorporated. Stir in the chopped tarragon.
Sprinkle the fish evenly with salt and pepper. Heat a large grill pan (or cook on your grill.....preferred) over medium-high heat. Coat the grill with cooking spray. Add fish, and cook for 5 minutes on each side or until the desired degree of doneness. Serve with the sauce.