Fabián is a member of the List of Counsel before the International Criminal Court. In this capacity, he has advised individuals and NGOs on matters ranging from victims' participation in the proceedings to the protection of privileged communications to issues of self-incrimination, in a number of cases.
Previously, he acted as legal adviser to the Kingdom of Saudi Arabia in the case of The Prosecutor v Radovan Karadzic, before the International Criminal Tribunal for the Former Yugoslavia (ICTY). As Associate Legal Officer of the Office of the Prosecutor of the ICTY, he was involved in the appellate proceedings of the first genocide cases before the Rwanda Tribunal (Akayesu; Rutaganda; and Kayishema and Ruzindana).
Fabián was the Secretary-General of Amnesty International Argentina from 1998 to 2000 and has been a member of the Colegio de Abogados de La Plata (Argentina) since 1990.