Place chicken breasts between two sheets of plastic wrap and use a meat mallet to pound to even thickness, about 1/2 inch thick. Remove from plastic wrap and season on both sides with salt and pepper.
In a large sauté pan, heat olive oil over medium-high heat. Add chicken breasts and cook for about 5 minutes per side, or until cooked through and no longer pink inside. Remove from heat and transfer chicken to a plate to rest for 5 minutes.
Cut chicken into bite-size pieces. Toss with buffalo sauce. Add any additional sauce to taste.
Use a kitchen shears or knife to remove the longer stems from the spinach. Chop the leaves a bit.
In a large salad bowl, toss together the spinach, celery, onion and avocado. Add the buffalo chicken and sprinkle with Cooper Cheese. Drizzle with blue cheese dressing. Toss to combine if desired.