My boss made fun of my stutter

My boss made fun of my stutter There was an incident where my boss talking to me and I was talking to her back and I get severe blocks on I's and D's but I've been working at this company for about 2 years and my boss knew about my stutter so I wasn't very self-conscious anymore so I stuttered when speaking to her. Then I heard from a fellow colleague that the next day she was mimicking my stutter and making fun of me. How do I go about this?