Use your leftover corned beef to make this revelatory hash, perfect for Sunday brunch. Four kinds of root vegetables, sauteed to caramelize them and bring out their sweetness, make the perfect counterpart to the salty corned beef.
In a 12-inch skillet, heat 1/4 cup of the oil over medium-high heat. Add the potato, celery root, onion, turnip, rutabaga, garlic and thyme, and cook, stirring occasionally, until the vegetables are golden-brown on all sides, 7 to 8 minutes.
Add 3/4 cup of the brisket cooking liquid. Cook on medium-high heat to reduce the liquid to a syrupy consistency, about 2 minutes. Add 2 Tbs. of the butter and stir until melted and well mixed with the vegetables. Remove from the heat and set aside.
Heat the remaining 2 Tbs. oil in a large (11- to 12-inch) cast iron pan or heavy-duty skillet over high heat. Add the corned beef in a single layer. Sear without stirring until it develops good color, about 3 minutes. Stir the beef and add 1 Tbs. of additional oil if the pan looks dry. Sear for about 3 more minutes to brown on the other sides.
Fold in the vegetables and the remaining 1/4 cup cooking liquid. Stir in the remaining 1 Tbs. of butter, and the parsley and chives. Cook over medium heat until the liquid is completely evaporated, about 5 minutes. Season to taste with salt and pepper.
The hash is delicious topped with a soft poached egg and hollandaise sauce.