To begin, “The Imagined Forest,” an orchestral piece by a young composer, Grace-Evangeline Mason; to end, Ravel’s orchestration of Mussorgsky’s fantastical and just as crowd-pleasing “Pictures at an Exhibition;” and in between, a recently rediscovered 20th century concerto by Thomas de Hartmann, featuring a soloist who has lately become a regular Heyward collaborator – the internationally acclaimed violinist Joshua Bell. No pressure! Thursday evening’s performance turned out to be one of the BSO’s strongest showings this fall, a feat that feels even more impressive because of the last-minute baton pass. Throughout the evening, Handler led with confidence and authority; one...