Possible Clues:

• Mint often used as an herb
• Herb is all for Cerberus and his work, by the sound of it
• Sounds as if Cerberus needed to get Herb
• Express disapproval at sound work produced by plant
• That's the herb that Cerberus had to get, by the sound of it
• Minty plant unknown in shops, oddly
• Aromatic mint
• Posy she made up mostly from these little blue flowers
• Plant with minty leaves used in cooking or as medicine
• An aromatic herb