In a skillet over medium heat, cook brats, onion, and garlic in oil until brats are browned on all sides.
Pierce brats with a fork and cook for 2 more minutes.
Spoon brat mixture into a slow cooker; pour beer over all.
Cover and cook on low setting for 4 to 5 hours, until brats are no longer pink in the center.
Serve brats on buns, topped with onion from slow cooker.