Mix oil, mustard, lemon zest, lemon juice, Worcestershire sauce, garlic, sugar and pepper in small bowl.
Pour 1/4 cup of the mustard mixture over chicken in nonmetal bowl; toss to coat. Cover. Refrigerate at least 1 hour to marinate. Refrigerate remaining mustard mixture until ready to serve.
Preheat grill to medium-high heat.
Drain chicken; discard marinade. Grill chicken 10 to 15 min. or until cooked through, turning once.
Cut into thin strips.
Toss chicken with lettuce, mushrooms, tomatoes and croutons in large bowl.