Pierce the skin of the bratwurst with a fork.
Grill sasuages for approximately 8 minutes until "just about" done.
Add 2 tablespoons of butter into a pot over medium heat until butter is soft. Add sausages and Oktoberfest beer. Cover and let simmer over very low heat for 20 minutes to 1 hour. The beer flavor will be stronger the longer you keep in the pot. The bratwurst will finish cooking during this step.
As the bratwursts are simmering, add 1 to 2 tablespoons of butter into a saucepan with onion. Cook until the onions are clear. Once the onions are clear, add the brown sugar and mix with the onions. Take off heat before the brown sugar is added.
Serve with hot Düsseldorf mustard on a bun with a bottle of Oktoberfest beer.