Heat the oil in a large skillet over medium-high heat and add the bratwurst links. Let them sear for a few minutes per side until deeply golden brown all over. Resist poking them with a fork here, since piercing the casing lets the juices escape and leaves you with dry sausage.
Once the brats are nicely browned, transfer them to a plate and set them aside for a moment. They won't be fully cooked through yet, and that's exactly the point, since they'll finish cooking gently in the beer later. This step is really just about building color and flavor.
In the same skillet, add the butter and sliced onions. Cook them over medium heat, stirring occasionally, until they turn soft and golden, about 5 to 7 minutes. All those browned bits left behind from the sausages will dissolve into the onions and add incredible depth.
Once the onions are softened, pour in both bottles of lager and stir everything together, scraping up any browned bits stuck to the pan. Bring the mixture to a gentle simmer. This beer-onion base is what turns simple sausages into something really memorable.
Nestle the browned bratwurst back into the skillet with the onions and beer. Reduce the heat to low and let everything simmer gently for about 12 to 15 minutes, turning the sausages occasionally, until they're cooked through and the internal temperature reaches 160°F.
Once the brats are done, you can remove them and let the beer-onion mixture reduce a bit longer if you'd like a thicker gravy. Spoon the onions and a little of the sauce over the sausages, then serve them hot with mustard and toasted rolls on the side.