Pour 2 cups strained broth from roast beef or beef stock into a slow cooker and set to high heat. In a pan, melt butter over medium-low heat. Once melted, whisk in the flour. Cook briefly, until combined and starting to bubble, about 3-5 minutes. Do not let the flour darken.
Slowly pour about 1 cup of the hot broth into the flour mixture, whisking well. Once smooth, pour the mixture into the slow cooker with the remaining broth.
Whisk to combine until smooth, then cover and cook over high heat until thickened, 30 minutes to 1 hour. Uncover and allow to cool slightly. Season with salt and pepper to taste. Serve warm with desired meal.
