Strain the pot roast juices into a bowl or measuring cup
Let the fat rise to the top
Skim off excess fat
Pour the defatted juices into a saucepan
Whisk 1 to 2 tablespoons cornstarch with 2 to 3 tablespoons cold water per cup of juices
Bring the juices to a simmer over medium heat
Slowly whisk in the cornstarch slurry
Cook while whisking until thickened
Add salt and black pepper to taste
Add a little beef broth or water if the gravy is too thick
Add a little more slurry if the gravy is too thin
Strain the gravy if you want it smoother
Serve warm