In a large skillet over medium heat, add 4 tablespoons of the butter. Once the butter has melted, add the cooked potatoes, diced onion, salt, and black pepper. Cook, occasionally stirring, until the onions have softened and the potatoes start to brown, about 8-10 minutes.
Add the cooked corned beef, garlic powder, and onion powder to the pan. Stir so everything is combined.
Press the mixture down with the back of a spatula to form an even layer. Cook for 3-5 minutes or until a golden crust begins to form on the bottom.
Scrape the bottom of the pan and flip up the hash so the crusty part is on top. Add the remaining butter, and mix it in.
Again, press the hash down using the spatula to form an even layer. Cook another 3-5 minutes until golden on the bottom.
Continue the browning step until you reach your desired color or crispiness. We usually do this 3 times.
Garnish with parsley if using, and serve with toast or other breakfast items.
Notes
This makes a large batch, but you can easily double it to feed more or halve the recipe as needed.
Tons of custom options, see my tips above on that.
Great use of leftovers!
Use cooked potatoes or make your own before making the hash.
This reheats and freezes well, see my tips above on how to store.
Use sweet potatoes in place of russets for a twist!