Add the beer and onions to a deep skillet or pot. Bring it to a boil over high heat. Stir in the Tony's seasoning. Add the sausages, then reduce the heat and simmer for 10 minutes (do NOT pierce the sausages - they will end up dry and overcooked).
Meanwhile, add the coleslaw, apple, and scallions to a salad bowl. In a smaller bowl, combine the dressing ingredients (mayo, sugar, lime juice, vinegar, mustard, and Tony's seasoning). Add the dressing and toss a few minutes prior to serving.
Preheat your BBQ/grill to medium-high heat.
Take the sausages out of the beer mixture and transfer to a plate (use tongs to do this easily). Grill over medium-high heat for 10 minutes, flipping once. I use a gas BBQ with the lid down.
If desired, toast the buns on the grill just prior to serving. Serve brats in the buns with the slaw on top.