Ask her out for lunch or something.
Or: is there a way you could, somehow, wind up in the same place for lunch? Perhaps you can say “mind if I sit with you?” Like you would for any work colleague. Maybe you’ll get lucky that way.
Or: if there’s a party for her leavetaking, you could ask if you could keep in contact. Or even if there isn’t a party, you could ask if you could keep in contact with her.
Come to think of it, the third option might be best.