Reserve remaining sauce. Drain marinade from fish into heavy medium skillet. Cook over high heat until beginning to boil. Add fish and cook until just opaque in center, about 4 inches per side. Combine watercress and radishes in large shallow bowl and toss to blend.
Arrange fish slices on top.
Sprinkle with sesame seeds. Serve with reserved sauce. Cover and refrigerate 1 hour, turning fish frequently. Remove fish from marinade, reserve. Place fish on grill over hot coals.