A simple casserole that is both delicious and keeps well frozen.
In a large skillet, melt the butter over medium-high heat. Add the onion, garlic, carrots, and celery. Cook until the onions are soft, making sure to stir often so it doesn't burn. Pour into a baking pan.
Add in the chicken, rice, soup, chicken stock, and salt & pepper to taste. Cover and bake at 425 degrees Fahrenheit for 30 minutes.
Leftovers can be kept in the freezer for up to 2 months.