Heat a large nonstick skillet over medium-high heat. Add sausages and brown on all sides, about 10 minutes.
Meanwhile, coat the bowl of a slow cooker with nonstick cooking spray. Place butter, brown sugar and mustard in slow cooker; stir in onions, caraway seeds, salt and pepper. Place sausages over onions and pour beer over the top.
Cover and cook on HIGH for 4 hours. Serve sausages on rolls with onions. Top with mustard and cheddar, if desired.
