Start by cracking the egg into a small bowl. Add the salt and ground black pepper, and whisk until combined.
Heat a non-stick skillet over medium heat. If desired, add the optional butter to the pan. Once melted, pour in the beaten egg. Let it cook for about 2-3 minutes until the edges begin to set. Gently flip the egg and cook for an additional minute, or until fully cooked. Remove from heat.
While the egg is cooking, toast your English muffin in a toaster until golden brown.
Spread the honey mustard on the bottom half of your toasted English muffin. Layer on the slices of corned beef, followed by the cooked egg, and top with one slice of Swiss cheese. Place the other half of the muffin on top.
Return the assembled sandwich to the skillet over low heat. Cover the pan with a lid to help melt the cheese. Cook for about 2-3 minutes, or until the cheese is gooey and melted.
Once the cheese is melted, carefully remove the sandwich from the skillet. Slice it in half if you prefer, and dig in!
