I'm actually mashing an onion beer right now! I roasted the hell out of two large red onions, and cooked down 3 more in a slow cooker with water, and added that to the mash. The slow cooked onions had turned a nice light brown, and the roasted onions were black on the outside with some carbonized sugar in the bottom of the pan, but tan to brown on the inside.
I added all the onions to the mash, with the hope that I'll get some color and flavor, but that the boil and fermentation will drive off the sulfury, oniony character and leave a much more subtle flavor behind in the final beer.
I will ferment with 2nd generation T-58 in the hopes that the yeast esters and phenols will blend with the subtle onion flavor, while a very active fermentation will help break down/drive off the overt onion character.
We'll see how it goes...hopefully well, since I'm brewing 5 gallons and kegging.
If not, I guess I have 5 gallons of marinade.