Pour beer into saute pan until it is about an inch high in the pan and turn to medium high heat
Stir the onions around until they are mostly covered by the beer. Simmer uncovered until all the beer has evaporated and absorbed into the onions, about 20-30 minutes.
Add butter to the onions and reduce heat to medium-low. Cook uncovered, stirring frequently 45-60 minutes until onions have caramelized and are a rich golden brown color.
Remove from heat and serve over the bratwurst. Onions can be gently reheated over medium-low heat in the pan if they cool too much before the brats are ready to serve.
Cooking the Brats
Fill the hopper with pellets, start the pellet grill, let it run through its start up process, then set the temperature to 225° F.
Lightly coat the skin of the brats in olive oil to keep them from drying out and splitting while cooking.
Once the pellet grill is up to temperature, put your bratwursts on the grill grates away from any direct heat. You can even place them on the upper rack above an aluminum foil pan of water. Close the lid.
Once the brats have reached 165° F internally using an instant read thermometer, you can remove them from the pellet grill, likely after about 1.5-2 hours.
If you wish to have a crispier exterior, you can throw the brats on a hot grill for 1-2 minutes per side just before they reach 165° F to finish them without overcooking.
Serve on a bun with the braised onions, or sauerkraut, mustard, or any other favorite condiment!