How to politely ask my girl friend to not make fun of my stutter?

So I have a stuttering problem, not a serious just moderate and my girl friend mocks my stutter a lot by repeating the words I get stuck on. For eg I was returning from market with groceries and she was standing with her friends besides the road so I crossed paths with her, she saw me and asked "Kya leke jaa raha hai?", I replied "Ssssabji" then immediately she retorted "Ssssabji" then giggled along with her friends, I didnt do anything - not turned my back, stopped, just kept walking ignoring her mockery. This is a real event that has happened with me and not doing any karma farming. We were together in school too and her behavior was exactly the same. People have mocked my stutter behind my back so it doesnt bother me but getting right mocked at my face is very hurting. I want to tell her to stop mocking because I find it very hurtful and it took lot of therapy, meds to accept my stutter. I also dont want to ruin my friendship by cutting ties with her, so I want some help as to how should I tell her this in a polite manner.