I'm trying to find a specific fic where Severus trains Harry, but I just can't remember which of the many it is.

In the story, Severus teaches Harry to brew his own potions, and identify poisons, and he gives Harry a box to keep his own potions in. Which is good since Draco puts poison on his food later, in the Great Hall, and he has a handy antidote. Dumbledore doesn't know about what Harry is being taught though, and when Harry asks Severus about it, he says something like "I wanted you to find me useful." That line is what sticks out the most for me. It might be the same one where Kingsley comes for a day and teaches him hand-to-hand combat, not sure though.

I know it's kind of vague, and there are a lot of training fics out there, but I'd be grateful if anyone remembers it. Thanks!

Found: Penance is the Play.
Thank you d_andru for remembering the fic even with so little info.