Like all reptiles and amphibians, turtles are not kosher. So turtle soup is not kosher (and also extremely uncommon nowadays).

How about mock turtle soup, made from calf meat? Provided that the meat is kosher, you would need to also be sure that there is no fish or dairy in the menu, since kosher-observant Jews do not mix meat with dairy or fish.

Another thing to be careful about: We are forbidden to serve food that closely resembles a non-kosher food. Classic examples would be meat cooked in almond milk, or a fake burger covered in cheese. These are allowed, however, if they are served in such a way that makes it clear the food being served is indeed kosher.

Back to mock turtle soup: In the 19th Century, turtle soup was an extremely popular delicacy, and there would be reason to fear that a person seeing mock turtle soup would assume it to be the real (non-kosher) McCoy. Thus, it would need to be served in a way that is visually different from turtle soup. Since authentic turtle soup is almost never served any more, there is good reason to believe that this would no longer be a concern when serving kosher mock turtle soup.