Line a 5- to 6-quart slow cooker with a Reynolds(R) Slow Cooker Liner.
Add beer, onions, maple syrup, vinegar, caraway seeds, thyme, and Worcestershire sauce to the prepared slow cooker. Stir gently with a rubber spatula to combine. Place bratwurst on top of the mixture in cooker.
Cover and cook for 4 hours on high or for 8 hours on low.
Serve bratwursts and onions in toasted buns topped with cheese and lettuce, if desired.