Add the beer (start with one can but make sure there is enough to cover bratwurst by about an inch; the pan size may cause the amount to be a variable) to a non-reactive pan along with the onions and butteer. Bring beer, onions, and butter to a boil. Drop in bratwurst and immediately add lid and turn off heat. Set timer for 7 minutes. When time is up, remove bratwurst, bring onions up to a good simmer and reduce as desired while the brats cook. Reduce for at least 10 minutes or until onions are tender or caramelize the onions by reducing longer until the onions are golden and most of the beer has evaporated, about 20 to 25 minutes.
Grill (or pan-fry; if pan frying, add a little oil to the pan) bratwurst using medium heat until nicely browned, about 6 to 8 minutes, turning and tending as necessary. If desired, brush cut sides of buns with a little oil and toast as the brats cook. Brats may be served as is or dropped into the pan with the onions.
Serve with the onions and mustard.
Keywords: Bargain Meal of the Week, Beer, Bratwurst, German, Grilled or Smoked, Pork, Sandwiches, Sausage