Use the empty thaumaturgy vessels on scorched guardian dragons found in the Badlands. Once you have them filled, bring them to Ghak Healtouch in Thelsamar.
You have been most helpful to me, [player]. I would like to reward you with the very recipe for the brew I gave you earlier, but I must ask of you a favor that is quite dangerous.
This lockbox contains three empty thaumaturgy vessels; they are imbued with an attunement aura that will drain the blood of a scorched guardian dragon. Using one on the beast will anger it greatly, so be careful. Once you have all three filled, bring them to me.
Do this, and the recipe will be yours.