Start by melting the unsalted butter in a large skillet over medium heat.
Add the sliced onions and cook until they're soft and translucent, about 10 minutes.
Pour in the beer, seasoning with kosher salt and black pepper.
Bring the mixture to a simmer.
Add the bratwurst sausages to the skillet, ensuring they are submerged in the liquid.
Cover and let them cook for 20 to 25 minutes over medium-low heat.
Preheat your grill to medium-high heat.
Once the bratwursts have finished braising in the beer, transfer them to the grill.
Grill the sausages for about 5 to 7 minutes, turning occasionally until they're browned and cooked through.
Serve with mustard on the side.