Cut the corned beef brisket, potatoes, and onions into ¼ inch cubes. Chop up the cooked cabbage to approximately the same size.
In a large skillet over medium-high heat add ½ of the butter.
Once the butter is melted add the diced onion and cook for 2-3 minutes or until they are translucent.
Add the potatoes and cooked cabbage to the skillet and stir to combine and coat everything in the melted butter.
Add the corned beef and salt and black pepper. Stir again to combine all the ingredients.
Reduce to medium heat and press the ingredient down in an even layer.
Add the remaining butter to the skillet, and allow it to melt.
After 2-3 minutes use a spatula and turn the hash revealing the crispy under crust.
Flip all of the hash over and press it down in the pan in an even layer in order to brown the top layer.
