Begin by heating 3 tablespoons of unsalted butter in a large skillet—cast iron works beautifully for even heating—over medium heat. Add the chopped onion and let it gently sauté for a few minutes until it becomes translucent and fragrant, releasing its natural sweetness. Next, stir in the chopped corned beef and potatoes, spreading the mixture evenly across the pan. Increase the heat to medium-high or high, then use a metal spatula to press down firmly on the hash. This helps create a deliciously crispy crust. Resist the urge to stir; instead, allow the hash to cook undisturbed until you hear a satisfying sizzle and the underside turns a deep golden brown. Carefully lift sections with your spatula to check the browning, then flip the pieces over to brown the other side, pressing down again to encourage crispness. If the hash begins to stick excessively, add a little more butter to the pan to help release those golden bits. Continue cooking and flipping until the potatoes and corned beef are evenly browned and crispy. Once cooked, remove the skillet from heat and gently stir in a generous handful of freshly chopped parsley. Season with plenty of freshly ground black pepper and salt to taste. Serve immediately, perhaps topped with a fried egg or alongside your favorite morning beverage for a satisfying meal.