“Steep one hollowthistle into a cup of boiling water and make him down the whole thing in one gulp.”
Hollowthistles are spiky red flowers sometimes used to make a vile tea. If seven cups, or, presumably, more, are downed in one sitting, the drinker's insides liquefy. After Fitz was injured in Neverseen, Physic (or Livvy) had him drink a cup of this tea every morning for a week. At the end of the week, he was healed.