Preheat the oven to 375°F and grease a 9x13-inch baking dish.

Bring a large pot of salted water to a boil. Cook the pasta according to the package directions, usually about nine minutes. Add the shredded cabbage for the last five minutes of cooking. Then drain both well without rinsing and set aside.

Melt one tablespoon of butter in a pan over medium heat. Add the diced corned beef and chopped onion, then cook for about five minutes, stirring occasionally until the onion softens. Set aside.

Melt four tablespoons of butter in a saucepan over medium heat. Whisk in the flour, salt, and black pepper, then cook for two minutes while stirring. Gradually whisk in the chicken broth and milk, then add the Dijon mustard and Worcestershire sauce. Keep whisking over medium heat until the sauce thickens and bubbles, then let it boil for one minute. Remove from heat and stir in the shredded Swiss cheese until melted.

In a large bowl, combine the drained pasta and cabbage with the chopped sauerkraut and corned beef mixture. Pour in the cheese sauce and stir until everything is evenly coated.

Spread the mixture in the prepared baking dish. In a small bowl, mix the Panko, melted butter, Parmesan, and optional parsley. Sprinkle the topping evenly over the casserole.

Bake uncovered for 18 to 22 minutes, until hot and bubbling with a golden, crisp topping. Let rest for a few minutes before serving.

Refrigerate leftovers in a covered container for up to four days, or freeze for up to two months. When reheating, add a splash of milk if the sauce needs loosening.