Preheat your grill for direct grilling at medium high.
In a saucepan, saute the onions with 2 tablespoons of butter until golden brown. Add the brown sugar, balsamic vinegar, oregano, salt, pepper and half of the beer. Simmer for 5 to 10 minutes and reduce.
While the caramelized onions simmer, add brats to grill. Grill 3 to 4 minutes on each side.
Serve brats in hoagie rolls with caramelized onions and mustard.