Preheat the oven to 350F. Cook the chicken breast and cut it into cubes. Try to make all the cubes roughly the same size.
Combine the stuffing mix with the melted butter in a large bowl—Reserve ¾ cup to use as topping for the casserole.
Add the veggies, cubed chicken, undiluted cream of chicken soup, and sour cream to the bowl with the stuffing and butter mixture. Mix all ingredients until everything is well combined so that every bite has a bit of everything.
Spray an 11 x 7-inch casserole dish with cooking spray (I like to use butter flavored), and spread the mixture evenly onto the dish. You don’t have to pad it too tightly. Just make it even on top.
Sprinkle the remaining ¾ cup of stuffing/butter mixture evenly on top and bake for 45 minutes or until it is bubbly and the topping is brown and crunchy.