Make vinagrette by mixing all ingredients well. Set aside.
POACHED FISH: In a medium skillet, bring 1-2 quarts water to simmer. Cut fish into 1" cubes and drop into water. Simmer 1-2 minutes. Drain carefully and allow to cool.
BROILED FISH: Cut fish into 1" cubes and place on a parchment lined baking sheet. Spray fish with butter flavored cooking spray if desired and sprinkle with lemon seasoning (such as Greek seasoning blend or Mrs. Dash). Broil 1-2 minutes watching so as not to burn. Remove from broiler and allow to cool.
Arrange salad greens onto a serving platter or individual salad plates. Carefully place the fish onto the greens. Gently spoon the vinagrette over top and serve.