BegoniaSpecies.org
B. crassicaulis, section Gireoudia, thick-stemmed group, is said to only bloom when grown under natural lighting. It drops its leaves in winter, when watering should be reduced. It is otherwise an easy grow species for garden or windowsill.
Crassicaulis flowers before new leaves emerge. Seeing this photo, I need to go hand pollinate before flowers begin to drop from "lack of love".