Bring a pot of salted water to boil. Place the potatoes in the boiling water and cook for 20 minutes. Drain and set aside.

Chop the bratwurst into pieces.

Heat one tablespoon of butter in a medium-sized skillet, add the spring onions and saute for a few minutes.

Add the bratwurst to the pan, mix to incorporate and fry for 5 minutes, until the bratwurst starts to turn brown. Remove from heat. Transfer the mixture to a plate and set aside. Reserve the pan.

Grease a casserole dish (I used a 10" round, 3" deep casserole dish)

Remove the skins from the potatoes and cut into thin slices. Place a layer of potatoes on the bottom of the dish, cover with the bratwurst/spring onion mixture, sprinkle half the cheese on top and cover again with a layer of potatoes.

Preheat the oven to 425 degrees F.

Heat the reserved pan. Add the remaining two tablespoons of butter.

Sprinkle the flour into the pan and stir it around.

Add the milk and the vegetable broth to the pan and bring to a boil, stirring. Reduce the heat and simmer for 3 minutes.

Season with Kosher salt and freshly ground pepper to taste.

Pour the sauce over the potatoes into the casserole dish.

Evenly distribute the remaining shredded Swiss cheese over the top. Place in the oven and bake for 25 minutes.

Remove from oven and let cool for 10 minutes before cutting and serving.  Garnish with extra spring onion pieces if desired.