Preheat a barbecue for medium-high heat. When hot lightly oil grate.
Combine the beer and onions in a large saucepan and bring to a boil. Submerge the bratwurst in the beer then add the chilli flakes, garlic powder, salt and pepper. Reduce heat to medium and cook another 10 to 12 minutes.
Remove the bratwurst from the beer mixture then reduce heat to low and continue cooking the onions.
Cook the bratwurst on the preheated barbecue turning once; 5 to 10 minutes.