Add butter, oil, corned beef and onions to a large cold nonstick or well seasoned cast iron skillet. Turn heat to medium low and cook, stirring occasionally, while you prepare potatoes.
In pan, boil potatoes in salted water for about 5 to 7 minutes, depending on the size, til partially cooked but still very firm. Drain very well and add to the skillet along with the rest of the ingredients except the chives.
Mix together thoroughly with the hot corned beef mixture and press down slightly with a spatula to flatten. Turn up heat to medium. Every 10 minutes or so, turn mixture over with a spatula to bring crusty bottom up to the top. Do this sev eral times til mixture is well browned. Take your time. The only real secret to great corned beef hash is to make sure it cooks long enough so the potatoes are crisp edges and cubes of meat get nicely caramelized. Taste for salt and pepper and adjust seasoning if needed. Transfer to plates, and top with poached eggs if desired. Garnish with freshly chopped chives.