Use the empty thaumaturgy vessels on scorched guardian dragons found in the Badlands. Once you have them filled, bring them to Jarkal Mossmeld in Kargath.
You have been most helpful to me, [class]. I would reward you with the very recipe for the brew I gave you earlier... but I would ask of you a task that is quite dangerous. Hear my request.
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 warned. Once you have all three filled, bring them to me.
Do this, and the recipe will be yours.