Get all of your ingredients together and pre-heat your STOK grill.
With your STOK cast iron kettle in place butter and sliced onion in your kettle. Pour the beer and water over it to cover. Don't have to use water. Bring to a boil.
Add your brats to the water and simmer for 30 minutes with your STOK kettle insert lid on.
Remove from heat and take the brats out of the liquid, reserve liquid. Place the brats on the grill and cook until golden brown.
You may now serve them with the mustard and onions.
Add sauerkraut if desired and serve.