Prepare and bake cornbread according to package directions, using a greased 8-in. cast-iron skillet. Cool on a wire rack.
In a small bowl, combine the tuna, mayonnaise, celery, onion, egg, dill, salt and pepper. Cut cornbread into 6 pieces; top each with ¼ cup tuna mixture and a slice of cheese.
Broil 4-6 in. from the heat until cheese is melted, 2-3 minutes. Top with tomato and avocado.
