Found a cockroach in our food at Ami Bengali restaurant, the staff alleged that we put the cockroach in the food

Yesterday, ie 21 Feb, me and my friends (college students) went to this bengali restaurant Ami Bengali in Nehru Enclave. We ordered a few items, which were priced comparable to most other expensive restaurants. When about to finish, one of us spotted a cockroach in one of the curries (video attached). We were disgusted, but quickly took a video of it and informed the staff. Two other customers had noticed what happened and left immediately while thanking us for letting them know. We didnt know what to do next, but were sure that we were not going to pay for this horrible experience. We explained this to the staff (calmly) and left the restaurant. It was only when we had walked away from the restaurant, a staff member drove to us, stopped us, and began a verbal spat in bengali (with a bengali guy in our group). His major points were: 1. We put the cockroach in the food ourselves to get free food. 2. Every restaurant has cockroaches, so getting one served is not a big deal. (So he agrees they have a cockoach infestation but at the same time alleges that we put it in the curry?) 3. We need to pay for every other dish that did not have cockroach. We ended up not paying anything, recorded the whole incident and would file a complain through FSSAI (hoping that they do something). Let me know what should have I done instead, and what should I do next?