There are no kosher restaurants in Monte Carlo. In Nice we went to Meat Bar which is kind of an Israeli style place with burger, schnitzels, salads. I think they had shawarma too, I'm not sure.
Then the other night we went to Le Dauphin Bleu which has pasta, meats, salads, etc. A bit more upscale but not too fancy.
Food in both places was very decent.
In Cannes we were only there during the day and while the dairy place wasn't open yet for the season, we found this fast food style shawarma/falafel place called Rouvi that was very good. They had bagels, hot dogs, burger, schnitzel, shawarma...small little place with just a few tables but food was very good.