Organ improvisation on a Russian folk song "Valenki" (David Cassan): Organ festival Q&A

Performing at the Mariinsky, French organist David Cassan was asked to improvise on a theme of a Russian folk song "Valenki" — and really succeeded at the task! Here is a fragment of that improvisation.
-----
In October the International Organ Festival took place at the Mariinsky Concert Hall for the ninth time. What kind of music can be played on organ? How is the instrument being tuned? What is special about the organ "Alfred Kern et fils", built for the Mariinsky Concert Hall in 2009? Learn about it with our short Q&A series:
https://youtube.com/playlist?list=PLxI0GHi7rfQLVK14Rzsn0m83KsvwQ8SHU