With my leftover corned beef, I always make Homemade Corned Beef Hash for breakfast the next day.
Chop your onion, then sauté with butter in a large pan or skillet until onions are translucent. Remove the pan of onions from heat.
Bring 8 cups of water to a boil. Peel, and dice potatoes to about ¼-inch cubes. Finely chop your corned beef until the pieces are the size of crumbled taco meat.
Cook until the meat gets nice and crispy, stirring and flipping the mixture often so as not to burn. It usually takes a good 20 minutes to get it to perfection.