Saute garlic in oil for a minute. Add ginger and saute another minute. Add the sliced beef, cook until mostly done. Remove the beef and set aside. Add the water and soy sauce and bring to a simmer. Dissolve in the brown sugar. Stir the sauce until it thickens slightly (add flour or corn starch if needed) and add the beef. Simmer and stir until sauce is just thicker than syrupy.