Moment of Impact by SuiteSambo
Genre: Mentor/Father!Snape and Mentee/Son!Harry
Rating: PG
This is a must read. For anyone in the Harry Potter fandom. It is written absolutely beautifully. The emotion, the description, the way that the story is so beautifully woven into canon will leave you astounded. The gentle progression of the father-son relationship between Harry and Severus is entirely believable. There is also some wonderful magical theory in the story, describing Occlumency and how it actually may work. The author also has a number of sequels to fit in between the last chapter and the epilogue of this story, which are also perfect. Mentor!Snape is one of my favorite tropes, and this is personally my all time favorite. It's long and satisfying, and you won't be able to stop reading it until you reach the end.
This can be found here.