Heat oil in a large skillet over medium-high heat. When the oil shimmers, add the bratwurst to the skillet. Sear the sausages until nicely browned, about 3-5 minutes per side.
Add the onion and beer. Increase the heat to high; bring the liquid to a boil.
Once the liquid boils, reduce the temperature to low.
Simmer over low heat, uncovered, until the liquid reduces, the onions soften, and the sausages reach an internal temperature of 160°F, about 8-10 minutes total. Flip the sausages halfway through, and occasionally stir the onions.
Serve the brats on the toasted buns. Garnish with red cabbage or sauerkraut, mustard, and parsley.