Pour the drippings from cooking a roast into a measuring cup and let the fat rise to the surface. Skim off the fat, reserving 4 tablespoons in a saucepan and discarding the rest. (If the fat measures less than 4 tablespoons, add enough butter to make up the difference.) Measure the defatted pan juices; if you have less than 2 cups, add water to make 2 cups. Boil the pan juices, the 2 cups water, and the onion until the onion is clear. Add the beef and return to a boil. Remove from the heat and stir in the evaporated milk. Serve over rice, grits, or cornbread.