Mix the salt, pepper, garlic powder, and Italian seasoning together in a small bowl. Sprinkle half of the mixture onto the chicken breast, and let the chicken breast sit at room temperature for about 10 minutes.
Heat the olive oil in a pan over medium heat. Cook the chicken for about 4-5 minutes per side, until browned and cooked through. Transfer the chicken to a large bowl and allow it to cool for a few minutes.
Slice the avocado in half, and discard the pit. Scrape the avocado flesh into a mixing bowl. Mash the avocado with a fork, then stir in the pesto, mayonnaise, green onion, celery, pecans, and the remaining half of the chicken seasoning blend.
Shred the chicken or cut it into small cubes, then mix into the avocado mixture. Squeeze in a wedge (or two!) of fresh lemon, and stir until everything is completely combined.
Divide the avocado chicken salad between two croissants, and serve immediately.