Hash.
Rid cold corned or roast beef of fat, skin and gristle, and mince it in a wooden tray with a sharp chopper until the largest piece is not more than an eighth of an inch square.
With two cupfuls of this mix a cupful of mashed potato rubbed smooth with a potato beater or wooden spoon.
Season well with pepper and salt if the beef be fresh, if corned use the salt sparingly and pepper well.
Set a clean frying-pan on the stove with a cupful of beef gravy in it from which you have skimmed all the fat. Clear soup will do if you have no gravy. If you have neither, pour into the pan a half-pint of boiling water and stir into this three tablespoonfuls of butter. When the butter-water (or gravy) reaches the boil, add a half-teaspoonful of made mustard.
Then put in the meat and potato and stir—scraping the bottom of the pan to prevent sticking—for five minutes, or until you have a bubbling-hot mass, not stiff, nor yet semi-liquid. It must have been brought to boiling heat and kept at it about five minutes, cooking so fast that you have to stir and toss constantly lest it should scorch.
Heap on a hot dish, and eat from hot plates.