Don't pack the stuffing. A loose, mounded layer stays light and crisps on top; pressed down it turns dense and gummy.
Frozen vegetables go in frozen. Thawing first releases water into the filling.
Nutrition, per serving (¼ of the pan): about 460 calories, 25 g protein, 3 g fiber. Assumes about 1 tbsp butter for the onion — using oil or less butter lowers it slightly.
Swaps. Cream of mushroom or cream of celery soup work in place of cream of chicken. Leftover shredded rotisserie chicken (about 2 cups / 280 g) replaces the canned chicken.