Place a large skillet over medium heat. Add 4 tablespoons of butter. to the pan. Once melted, add the cooked potatoes, onion, salt, and pepper. Cook, stirring occasionally, until the onions are softened and the potatoes and onion start to brown, about 8-10 minutes.
Add the corned beef, garlic powder, and onion powder to the pan. Stir everything together. Using the back of a spatula, press the corned beef mixture down in the pan 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 the hash over, working in sections, so the crusty part is on top. Add the remaining 2 tablespoons of butter and mix it into the hash.
Using the spatula, press the hash down into the pan again to form an even layer. Cook for another 3-5 minutes until golden on the bottom. Continue doing this until you reach your desired browning. (I usually do this a total of 3 times.)
Serve warm with a sprinkle of chopped parsley on top for garnish. I usually serve corned beef hash with sunny-sid-up or over easy eggs and toast.
Notes
If you don’t have cooked potatoes on hand, just dice up peeled russet potatoes until you get 4 cups. Then boil them just until fork tender, 5-10 minutes. Drain well and continue with the written recipe.
If you're using salted butter, I would start out with half the salt and season to taste at the end.
You have to add more butter in step 3 because the pan can tend to get dry. The additional butter helps the hash continue browning.