Dean and Sam are caught in the "French Mistake"/Meta!Universe and Misha is in a relationship with Jensen. Assuming Dean is his Jensen, Misha takes him to their home and cares for him etc. Dean gradually falls in love with Misha and more so, Cas because he understands how perfect Cas is compared to everyway Misha is perfect for Jensen.

There are two specific things which define this fic and may help narrow the search:

Misha buys Jensen a car that is exactly like the Impala, right down to the S.W and D.W engravings (not knowing that he's giving it to Dean, not Jensen) 
and
Dean takes a picture off of the fridge where Misha and Jensen are looking very happy with each other, before he returns to Cas. When back in the SPN universe Cas finds the photo and realises Dean loves him.

I hope this helps.