When preparing bulk roast dinners for the freezer, flour is significantly superior to cornstarch. Cornstarch breaks down during the freezing and thawing process, resulting in a spongy or watery texture. Flour, when cooked into a traditional fat-and-flour roux, stabilizes much better.
For a standard gravy, the ideal ratio is 2 tablespoons of fat and 2 tablespoons of flour per 1 cup of liquid. Whisk the flour into the melted fat and cook for a minute before slowly streaming in your broth to completely prevent lumps.