Don't bother with lager. The best cooking beer is ale. It's also the best drinking beer. You can also wash your hair with it. Horses will drink ale.
Ale is also a medicinal drink.
But of course you knew that. Wash, peel and chop the Brussel sprouts, and boil them for 15 minutes. Mash it all up in a large bowl. Add some salt and pepper. Stir in the beer.