Prepare the ingredients: Dice your cooked corned beef, potatoes, and onion into a uniform 1/2-inch dice. If using raw potatoes, place diced potatoes in a pot of cold salted water, bring to a boil, and cook for 5-7 minutes until just fork-tender. Drain well and let them cool slightly.

Heat a large 12-inch cast-iron or non-stick skillet over medium-high heat. Add 1 tablespoon of oil. Once shimmering, add the diced onion and cook, stirring occasionally, until softened, about 5 minutes.

Add the remaining 1 tablespoon of oil, the diced potatoes, and the corned beef to the skillet. Gently mix everything together. Add the butter, Worcestershire sauce, and black pepper. Stir to combine.

Using a spatula, press the mixture down firmly and evenly across the bottom of the pan.

Reduce heat to medium and cook, undisturbed, for 7-9 minutes. Allow a deep, golden-brown crust to form on the bottom before touching it.

Use a wide spatula to flip the hash in large sections. Press it down gently again and cook for another 5-7 minutes, or until the other side is browned and crispy.

Stir in the fresh parsley. Serve hot, topped with a fried or poached egg if desired.