Bbm11/C# (Bb Minor 11th over C#)
Notes, from the bass up
C# (bass)E#G#B#D#A#
A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read Bbm11/C# as Bb Minor 11th with C# in the bass.
The bass note here is one of the chord tones, so this is exactly the 1st inversion of Bb Minor 11th, written the way sheet music and chord charts usually write it.