Trim any tough casing or loose ends from the bratwurst. Peel the onion and cut into 1/4-inch thick rings or half-moons. Measure out beer, butter, oil, and other ingredients so everything is ready.

Sear brats

Heat a large skillet or wide sauté pan over medium-high heat. Add the vegetable oil. When the oil shimmers, add the brats, spacing them so they don’t touch. Sear the brats, turning every 2–3 minutes, until all sides are nicely browned (about 6–8 minutes total). Do not pierce the sausages while searing.

Sauté onions

Push the seared brats to one side of the pan (or remove briefly to a plate if your pan is crowded). Add the butter to the empty space in the pan. When it melts, add the sliced onions and a pinch of salt. Cook the onions over medium heat, stirring occasionally, until softened and starting to caramelize, about 6–8 minutes. Sprinkle the onions with the apple cider vinegar and stir to deglaze the pan and lift any brown bits.

Simmer in beer

Return brats to the pan if you removed them. Pour in the WDBC beer so it comes about halfway up the sides of the sausages (use up to the specified 24 fl oz; you may not need it all depending on pan size). Bring the liquid to a gentle simmer, then reduce heat to medium-low, cover the pan, and simmer for 12–15 minutes. This finishes cooking the brats through and infuses them with beer flavor.

Finish and season

Remove the lid and continue cooking uncovered for 2–3 minutes to reduce the braising liquid slightly and concentrate flavor. Taste a spoonful of the onion-beer sauce and season with kosher salt and freshly ground black pepper to taste (about the amounts listed). Stir in 2 tablespoons of brown mustard, reserving the remaining mustard for serving if desired.

Toast buns and assemble

While the liquid reduces, split the buns and lightly toast them cut-side down in a separate dry skillet or on a grill for 1–2 minutes until golden. Place a brat in each bun, spoon several tablespoons of the onion-beer-pan sauce and caramelized onions over the top, add a dollop or spread of the remaining brown mustard, and top with sauerkraut if using.

Serve

Serve the brats hot with extra mustard on the side and remaining beer for drinking. Leftover brats can be refrigerated in their cooking liquid for up to 3 days.