In a can cooker, cook brats according to package directions; remove and keep warm.Heat oil until hot and smoky. Add onions, cook and stir with a wooden spoon for 5 minutes.Add 1/3 of beer; cook and stir to reduce liquid.Add brats and another 1/3 of beer.Cook and stir to reduce liquid; add remaining beer and butter.Cook and stir to reduce liquid.Serve onions and brats on buns.