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I'm pretty sure I'm in the minority, but I've never found that beer or boiling of any kind enhances a REALLY GOOD brat. In fact, if you start with a great brat (Johnsonville Original is my favorite, but some of the local German places make some good brats as well) I think that cooking them in beer essentially just waters them down. This was reaffirmed for me last weekend at a friend's house. So, I like to cook them raised direct at 275 - 300. Any higher than that and they tend to burst. By exposing them to radiant heat while keeping the temp on the low side, they tend to brown nicely on the outside while cooking to doneness on the inside - without bursting if you take them off at the right time.
I am certainly not anti-beer or anti-onion in general, but I haven't seen them improve a good brat.