Prepare Herb Butter and set aside. Combine ingredients for poaching liquid in a fish poacher or large, shallow pan. Bring to a boil. Partially cover, reduce heat and simmer 20 minutes.
Gently place fish in liquid, submerging it entirely. Cook, uncovered, until fish flakes easily when tested with a fork, about 10 minutes. Gently remove to serving platter. Serve with Herb Butter.
In a small bowl, cream together butter, chives, parsley, tarragon, salt and pepper. Allow to stand at least 1 hour to develop flavors. Spread over warm fish.