Mash 1/2 the potatoes with a fork in a large bowl until smooth, add the remaining potatoes and the corned beef,stock,parsley, mustard,thyme, onion, garlic,nutmeg and pepper- mix until evenly combined. Heat the butter in a 12-inch cast-iron skillet over medium-high heat. Add the corned beef mixture and cook, stirring constantly, until lightly browned, about 1 minute. Using a large spatula, press the corned beef mixture into a flat cake, and cook until the mixture is browned and crusty on the bottom, for about 10 minutes. Flip the cake, and continue cooking until the mixture is crowned and crusty on other side, about 8 minutes. Transfer to a serving plate and cut hash into wedges to serve.