PREHEAT grill to medium-high heat. Brush chicken with 1 Tbsp. of the dressing.
GRILL chicken 5 min. on each side or until cooked through (170°F). Meanwhile, grill bread slices 1 min. or until lightly toasted on both sides, turning over after 30 sec.
MIX tomatoes, cheese and basil; set aside. Toss spinach and onions with remaining 1 Tbsp. dressing; spoon onto serving plate. Top with cooked chicken and the tomato mixture. Serve with the bread.