Heat the oil in a large, deep, non-stick skillet (that has a tight fitting lid) over medium heat.
When the oil is hot, but not smoking, add the bratwurst. Cook the sausages 5 minutes turning to brown on all sides.
Carefully, add the beer and bring to a simmer.
Cover the skillet and lower the heat to medium-low. Cook for 20 minutes.
Uncover the skillet and raise the heat to medium-high. Allow the excess liquid to cook out, about 7-10 minutes. Turn the sausages occasionally to keep them from burning.
Take the skillet off the heat, set the beer brats on a plate or pan and keep warm.
Drain most of the excess oil in the skillet; leave about 1 or 2 tablespoons. Don’t wash the skillet.
Place the skillet back over medium heat and add the onions. Cook until they are softened and turn a nice golden color, about 5 minutes, stirring occasionally.
Serve the beer brats on the buns and top with a generous serving of onions, and mustard, if desired.