Dijon mustard (optional)
Brown the sides of the brats in a skillet on the stove. (This will let them still have a nice coloring afterwards like the grill).
Meanwhile, slice your onion.
If desired, mix some apple juice with a couple spoonfuls of Dijon mustard.
Place a layer of onions on the bottom of the crockpot. Top with the brats, and then place the rest of the onions on top. (If desired you could also slice up a green pepper to put in here as well. I personally like it with peppers and onions, but my husband is a brat purist).
Cover the brats with apple juice*.
Put on lid and cook in crock pot for 5-6 hours on low or until juices runs clear. Because you browned them beforehand you may not actually cook them that long.
Remove when done and serve with crusty or hogie buns and whatever condiments you like.
*There are plenty of recipes out there for cooking brats in beer in a crockpot. I don't drink beer, so therefore this is with the apple juice substitution.
Source: Recipe Shoebox as Bratwurst in a Slow Cooker.
Note: I haven't made these yet but a friend made them for a church function and they were a hit.