I know how it feels as I've been asked the same question before but I do mention to the employers that as a full stack how can you expect me to fully remember every bit of coding for front-end and back-end and in several languages? It's almost impossible!
I'm the kind of developer who has books by my-side to reference if I need to re-go over something about it :) But in regards to what you're asking. This link will really help you, and point you where you need to go for the backend. I would recommend at looking at Python and using the framework Django (full stack framework) and you'll learn that really fast.
Let us know how it goes ;) p.s this road map is updated every year for full stack developers according to trending knowledge's
Mark