Blend first 7 ingredients in blender until smooth.
Pour 1/3 cup sauce into glass pie plate, add fish and turn to coat, 15 minutes per side.
Reserve remaining sauce.
Drain marinade from fish into heavy medium skillet.
Cook over high heat until liquid starts to boil.
Add fish and cook to desired doneness, about 3 minutes per side for medium rare.
Combine watercress and radishes in bowl, arrange fish slices over
it, sprinkle with sesame seeds and reserved marinade.