Combine first 6 ingredients in a large bowl. Reserve half of marinade.
Add chicken to marinade in bowl, turning to coat. Let stand 5 minutes.
While chicken marinates, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Drain chicken; discard marinade. Add chicken to pan; cook 4 minutes on each side or until done.
Remove pan from heat; add reserved marinade and onions. Toss to coat.
Divide watercress evenly among 4 plates. Divide chicken mixture evenly among servings; top evenly with radishes and sauce.