Heat 1 tbsp of butter over medium heat in pan with sides high enough to hold onions and braising liquid.
Sear brats on each side, about 3 minutes, or until a nice browned mark appears.
Remove brats from the pan; melt 2 tbsp of butter over medium heat.
Add sliced onions and sautee until just beginning to caramelized (very soft, and they start to smell sweet). Add butter as needed to keep onions moist enough to be caramelized and not brown too much.
Add sliced apples and satuee until just softened.
Add cider and beer, scraping up any brown bits on the bottom of the pan. Bring to a boil.
Add 2 tbsp of dijon mustard and stir to combine.
Add brats back to the mixture, lower to a simmer and put cover on pan. Allow to cook for about 20 minutes or until brats reach internal temp of 160 (refer to brat package for exact internal temperature required).