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.
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.