I have never seen this kind of symbol before and google has failed me.
This notation occurs in Chopin Nocturne Op. 9, No. 1, measure 2 beats 4–6.
It's an 11-tuplet. Like a triplet, but with 11 notes where there would otherwise be 8 (or some other power of 2).
According to Wiktionary, the word to denote this is undecuplet or hendecuplet, although there are no articles for those words as of 8 June 2021.