In a large skillet on medium heat, add butter, oil, and onion. Cook until onion is translucent and starting to brown, about 5 minutes.
Add the potatoes and corned beef, then mix together and spread in the skillet. Increase heat to medium high. Cook without stirring until browned, checking occasionally, about 5-8 minutes.
Flip or stir to brown the other side and add additional butter if needed. Cook for about 5 minutes, and stir to brown the hash.
Sprinkle with parsley, pepper, and salt to taste.
Serve with a cooked egg and toast.
Notes
Add leftover cooked carrots and cabbage (chopped), or saute the cooked carrots and cabbage (chopped) and serve on the side.
Refrigerate leftover corned beef hash in a covered container up to 3 days, or freeze in a zippered bag for up to 2 months.