Make the Sauce

1. In a saucepan add in 4 Tablespoons of butter and once it melts, add in the flour. Whisking to combine and cooking for a few minutes to cook off that raw flour taste.
2. Slowly pour in the milk mixture constantly whisking to prevent lumps. Next, add in the salt, pepper, mustard powder, and yellow mustard whisking to combine. Allow the mixture to start to thicken to where it coats the back of a spoon (about 5-6 minutes).
3. Remove the hot béchamel from the heat. I tend to remove the pan from the heat completely when adding the cheese to prevent it from getting too hot and curdling/breaking.
4. Add 10 ounces of cheese slowly. Really, add in a little bit at a time. Add some, stir till melted and add more. Continue to add and stir.

Assemble The Casserole & Bake

1. Preheat the oven to 400F, rack in the middle. To a large bowl, add the gnocchi, leftover Guinness Corned Beef, and Sauerkraut (with optional caraway seeds). Gently stir to combine.
2. Pour in all of the mornay sauce and gently mix it up, coating all pieces.
3. Transfer to a 3.5 or larger quart baking dish and cover with foil. Bake in a preheated oven for 25 minutes. After 25 minutes, Remove from the oven and cover with the remaining Swiss cheese.
4. Mix together the 4 Tablespoons of melted butter with the panko, coating evenly. Sprinkle this on top of the casserole and return it to the oven for another 10-15 minutes or until the panko is golden brown.
5. Remove from the oven, allow to rest for 10 minutes, and enjoy!