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