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.
Notes
If you prefer to grill the bratwurst sandwiches, follow this recipe for grilled beer brats. You can even prepare the grilled sausage on an indoor grill pan.
For an alcohol-free recipe, substitute chicken broth for the beer. You can also use apple cider, but it will give the sausages a sweet flavor.
Add Swiss cheeseor sharp cheddar to the sandwiches, or try some crispy bacon for even more flavor.
To serve a larger group, double all of the ingredients. You may need to use two separate skillets (or work in batches) so that you don't overcrowd the pan.
Any variety of beer that you enjoy will work. Try a deep, dark beer for a stronger flavor (like Guinness or other stouts), or look for a lighter lager (such as Coors Lite or Miller Lite) for a more mild, kid-friendly taste. To make it authentic, pick a German beer!
Skip the bread and sandwiches, and instead serve the brats with mashed potatoes or spaetzle, or alongside big soft pretzels.
Simmer the brats uncovered to allow the beer in the pan to reduce, or cover the skillet to help the sausages cook even faster.
Total cooking time will vary depending on a number of different factors, so an instant-read meat thermometer is always the best way to know when your sausages are done. Bratwurst are considered safe for consumption when they reach an internal temperature of 160°F.