Way to unlock: Talk to Sera in Skyhold, you must be in relationship with her.
The quest-giver: Sera (M12,16)
After starting a relationship with Sera and while having a high approval level, when you will go to talk to her a cutscene will start, in which you will learn of the gift from the girl. Wanting to repay her, you must learn of possible options by talking with all companions: Iron Bull, Cole, Vivienne, Cassandra, Blackwall, Solas, Varrik and Dorian (unless you don't have all of companions).
After that return to Sera and talk to her. After another cutscene the quest will end.
No rewards for completing this quest.