What is this celery like vegtable?
It is an asparagus chicory, or more formally known as Puntarelle. They are grown in Rome and in Italy. They are similar to endives and have that bitter taste. A famous dish that uses it is Puntarelle alla Romana, a salad with anchovies and the asparagus chicory. It is quite hard to find outside of Italy, surprised you found it in California. Enjoy it!