Combine the coriander, lime juice, olive oil, salt, and garlic powder in a large zipper-close bag. Add the thin chicken breast pieces and allow them to marinate for 5 minutes.
Meanwhile, heat a large cast-iron skillet over medium-high heat. Discard the marinade and place three of the chicken pieces into the preheated pan. Cook for 2-3 minutes per side until golden brown and cooked through. Remove the cooked chicken and set aside. Repeat with the remaining three pieces of chicken.
While the chicken is cooking, prepare the salsa by combining the tomatoes, onion, lime juice, salt, and pepper in a large bowl. Use a wooden spoon to gently fold in the diced avocado. Arrange the cooked chicken pieces on a platter and top with the salsa. Serve hot.
