Combine first 4 ingredients in a small bowl, stirring well. Spread mayonnaise mixture evenly on cut sides of baguette halves. Arrange basil leaves over mayonnaise mixture. Layer chicken, onion, and cheese on top of basil. Broil 3 inches from heat (with electric oven door partially opened) 2 minutes or until cheese melts.
Cut each baguette half into 3 equal pieces. Serve immediately.