I don't think this can refer to the real bottarga which is cured fish roe, but wonder if might refer to a bottarga-like substance made of mature, purple carrots.

You can find several examples of chefs doing similar things with caviar so you can find fruit caviar, carrot caviar etc. I have chosen caviar rather than bottarga as people tend to know what it is and both are made of fish roe and both are expensive and regarded as a delicacy.

https://www.instructables.com/id/Carrot-Caviar/

I have only put a low confidence level as this is just a hunch. You would obviously need to check this with your client (or chef)

I would not translate mature as it doesn't make it sound very appealing, as mature carrots tend to be woody (I am a gardener)