In a large food storage container, combine the brats, onions, and beer. I used Next beer by Budweiser, which is a low-carb beer. Cover and marinate overnight.
When the brats have marinated, remove them from the container, reserving the onions and liquid for now, and place the brats in a skillet. Alternatively, you could grill them.
Cook on all sides Remove the brats from the pan and set aside.
Strain the onions from the marinade.
Add a drizzle of olive oil to a skillet.
Place the onions in the same pan where the brats were cooked. Saute until browned.
In a separate pan, melt some butter until it just starts to sizzle. Place the sandwich rolls inside down and brown in the butter until crispy and slightly toasted.
Serve the brats and onions on the hot toasty sandwich rolls.