Remove the cooked roast from the pan, and pour the pan drippings into a glass jar and let settle for a few minutes. Skim off the fat that rises to the top with a spoon, then place back into the pan.
Place pan on stove over medium-high heat, and bring to a boil.
Dissolve 2 Tbsp of cornstarch in 2 Tbsp cold water, then pour into pan - whisking constantly until the gravy begins to thicken.
As it thickens, slowly add the wine or water or stock, (or some combination of these) to the pan.
Season to taste, then continue simmering for 5 minutes.