Roll outside of peeled avocados in lemon juice to keep color. Put in large baking pan. Sprinkle with lemon juice and garlic powder. Melt butter; add green onions, celery salt and pepper. Cook 5 minutes. blend in flour. Add cream. Cook while stirring until thickened. Fold in chicken and 1/2 cup cheese. Bake avocados at 350 degrees for 15 minutes. remove to serving plates. Ladle filling over avocados. top with remaining cheese.