In a skillet over medium heat add the oil, butter and onions. Cook until lightly caramelized, about 15 minutes. Remove from pan, set aside.
Liberally salt and pepper the brisket on all sides.
Sear the brisket in the onion pan until browned on all sides. Add to a slow cooker. Top with onions and garlic, then add the beer, Worcestershire sauce and balsamic.
Cook on low for 6 to 8 hours or until fork tender.
Remove the brisket, thinly slice.
Drain the onions, transfer to a small bowl.
In a small bowl stir together the sour cream, horseradish, garlic powder and smoked paprika.
Fill the buns with brisket, onions and sour cream mixture.