The Pittsburgh Jazz Festival organized on June 20, 1965 a "Jazz Piano Workshop" which was luckily recorded and issued on CD by Mosaic Records: https://www.discogs.com/Various-The-Jazz-Piano/release/5260834. Here is a picture of the "cast" on that day: https://collection.cmoa.org/objects/cfcdaea2-de64-4f61-92cf-8f4e6d9db60f
Ellington plays two numbers (in addition to a duo with Earl Hines): a spirited version of "Take the A Train", with bass and drum accompaniment, and another piece "The Second Portrait of the Lion", an homage to Willie "The Lion" Smith, who was also performing that day. This is the outstanding piece on the album.
The first "Portrait of the Lion" was originally recorded in 1939 with the full band. This second portrait has little in common with the first. There are a few other performances of The Second Portrait recorded in concert (Paris, Italy and Denamark, 1967), but in shorter versions. This one is 4 minutes long, and really fascinating.
Ellington starts off with a few introductory chords that hint to a melody, as if he had just sat down to compose. Humming along (as The Lion often did), he jumps into a very "classical" stride theme, which he then proceeds to "deconstruct" only to jump back into the same stride theme a second time, to "deconstruct" it once again but this time moving into a slower solo (at 1:40) that lasts for over a minute and a half (up to 3:00). In this long interlude, Ellington is exploring: he hints back to the stride theme at various points, leads us into different melodies, constantly changing, as if he were composing "live" (this was certainly improvised) - everything is in motion. The stride theme is then played a third time, but this time ends in a series of chords (at 3:15) that sets us up for another slow ending, more coherent, poised, and in which the stride theme seems (to me) perfectly merged into the slow melody, as if to say that the pianist had succeeded in this transformation and found his inspiration and peace.
This is a brilliant homage of Ellington to the pianists that inspired him in his youth, with a perfect balance between classical and modern, that transcends any categories. As with anything published by Mosaic Records, the sound quality is good !