Once the butter is melted, add the brats. Sear brats in the dutch oven until they develop a nice crisp browning on the outside of the casing. This should take about 3-4 minutes. Turn the brats and sear on the other side.
While the brats are searing on the second side, add the onions to the dutch oven, stirring to soften.
Once the brats are browned on both sides, reduce the heat to medium and add the lager, 2 more tbsp butter, garlic powder, black pepper, and red pepper flakes.
Allow the brats to simmer in the beer for about 25 to 30 minutes or until the liquid is reduced to half and is dark golden brown.
Serve in a delicious Pretzel Rolls or brat bun with mustard.