Heat a large thick-bottomed pan (preferably cast-iron skillet). Add oil. Sear the brats on medium-high heat for about 3 minutes, flip using tongs and cook on the other side for another 3 minutes.
Add the onions between the sausages and cook for about 2 minutes, until golden, stirring a few times.
Flip the sausages again, add beer, cover, and cook on low heat for about 10 minutes, flipping again halfway. The internal temperature should read 71 degrees Celsius/160 degrees Fahrenheit. If you don't have a meat thermometer, just cut one sausage in the middle, it's easy to see if it's cooked through.
Remove the brats, add salt and pepper to taste, and reduce the sauce for about 2 minutes. Return the sausages to the pan and turn them into the sauce.
Serve immediately.
Video
Notes
If your brats are larger is fine, just increase the cooking time slightly and check if they are done by cutting one in the middle.