Preheat your grill to 350 degrees. Grill the bratwursts for 7 minutes on each side or until they reach an internal temperature of 160 degrees.
Add the onions to the base of the slow cooker. Add the grilled bratwursts. Pour the beer over them and then add 2 cups of water. (Please note that the brats just need to be covered in liquid).
Set the slow cooker to warm and let the flavors combine for 30 minutes. Then allow your guests to eat at their leisure, pulling out a Beer Brat and topping with with some of the onions in a bun.
Video
Notes
I've had some readers have a concern that the onions will not soften in just 30 minutes of cooking. They do get quite soft, though they have a small bite to them. If you are concerned about this, you could sauté the onions in butter first.