Preheat the oven to 400 degrees F. Rinse the chicken briefly under cool running water, then pat dry with paper towels. Season on both sides with 2 teaspoons salt and 1/2 teaspoon pepper.
Heat the olive oil in a 12-inch ovenproof saute pan over high heat. Add the chicken and cook 2 minutes. Turn the chicken over and immediately place the pan in the oven. Roast 10 to 15 minutes, or until the chicken reaches an internal temperature of 165 degrees F when tested with an instant-read thermometer. Remove from the oven and set aside to cool.
In a large mixing bowl, combine the mayonnaise, garlic, mustard, celery, celery leaves, red onion, herbs, celery seeds and cayenne pepper. Mix well.
When the chicken is cool enough to handle, cut it into a 1/2-inch dice, add it to the mayonnaise mixture and mix well. If not serving immediately, transfer to the refrigerator to chill. Serve in avocado halves, if desired.