Add 2 cups of the beef broth (or cooking liquid) to a small sauce pan. Heat over medium-high heat until the broth begins to simmer. Reduce the heat to medium and continue to simmer.

In a measuring cup, whisk together the remaining cup of beef broth* (or cooking liquid) and the flour until the mixture is smooth and (mostly) free from lumps. Slowly pour the mixture, which I recently learned is technically called a slurry, into the simmering broth, gently stirring as you pour. Continue to simmer and stir until the broth thickens into gravy. Remove from heat. 

To assemble the sandwiches, lay a slice of bread on the plate. Place a spoonful of mashed potatoes on the bread, followed by the desired amount of leftover roast, and topped with the delish brown gravy you just made. Enjoy!