1.Melt butter in large, heavy skillet (big enough to fit 4 fish fillets in single layer). Place fish, wine, broth and tarragon in skillet, bring slowly to a boil; reduce heat and simmer gently, covered, 15 minutes or until fish is just tender and starting to flake.
2.Meanwhile, heat an oiled saucepan; cook onion until soft. Add tomato, apple and cucumber; cook, stirring occasionally, 7 minutes or until apple is just beginning to soften.
3.Serve fish fillets with a little of the poaching liquid and the apple and cucumber salsa.