Heat a heavy based frying pan over a medium low heat.
Prick the bratwurst once each, and place the oil into the pan, followed by the sausages. Cook until golden brown.
Shuffle the brats off to one side of the pan, and add the butter, followed quickly by the onions. Stir them around to coat them in the butter, and then shuffle the sausages in and around the onions.
Cook until the onions are soft and beginning to darken and brown.
Add the beer to the pan, and cook gently for about 10 minutes, or until the beer has cooked away.
Serve the brats hot from the pan, stuffed into crusty bread rolls and slathered with hot mustard.