I mix my flour and cornstarch together and add some cooled broth to it until it is like pancake batter thickness? I drizzle in as much as I need to the boiling stock until it thickens, stirring constantly with a whisk so it doesn't stick. You could probably use milk to add to the flour/cornstarch mixture before adding it to the stock if that is your preference. I haven't had my gravy liquefy later, in fact after refrigerating it usually needs a bit of moisture to loosen it up.