Stirring up the past in time with the present. (слэш)

Harry Potter goes to work for Severus Snape after the war ends. But reminisces on the past while doing so. Specifically his 6th Year at Hogwarts.
Brewing for one Severus Snape, Potions Master and owner of Snape Potions was not for the faint of heart. And most definitely not for the squeamish or easily affected by barbs or verbal take downs, or for the imperfect.
You had to be a certain type of person to work for the Potions Master. And Harry Potter was that person.
 
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Written for the Prompt: Toxic.