- Stir 1 tsp salt, 1/4 tsp black pepper, and cayenne pepper together in a small bowl; rub evenly into the chicken breasts.
- Heat the olive oil in a skillet over medium heat; cook the chicken in the heated oil until no longer pink in the center and the juices run clear, 5 to 7 minutes per side.
- Stir the onion, lime juice, and avocado together in a bowl, season with salt and pepper. Spoon over the chicken breasts to serve.