MIX mustard and mayonnaise until well blended. Set aside
RUB chicken with Cajun seasoning. Preheat grill to medium/high; cook chicken 5 to 6 minutes on each side or until chicken is cooked through (170°F).
FOR each serving: Spread cut sides of each croissant with 1 Tbsp. of the mustard mixture.
PLACE 1 chicken breast, 2 bacon slices, 1 lettuce leaf and 3 tomato slices on bottom half of croissant; cover with top half of croissant. Cut crosswise in half to serve. Secure each half with 4-inch frilled toothpick.