Pull the chicken meat from the bones, discarding the skin. Cut the meat into bite-size pieces and transfer to a large salad bowl.
In a small bowl, combine the mustard and lemon juice and whisk to blend. Gradually whisk in the olive oil. Mix in the basil, shallot and chipotle chilies with their sauce. Season the dressing with salt and pepper. Stir 1/4 cup of the dressing into the chicken and let marinate while preparing the remaining ingredients.
Stir the cucumbers, tomatoes and bell pepper into the chicken, then stir in the lettuce. Add enough dressing to lightly coat the ingredients. Taste and adjust the seasonings with salt and pepper. Gently stir in the avocado and serve immediately. Serves 4.