To thicken a fruit coulis you will need to briefly cook it so that a thickener such as cornstarch, arrowroot starch, or kudzu will do their jobs and thicken. Start by making a "slurry" (starch mixed with a little cold water) until very smooth. Then, add to a small pan of hot coulis and cook for a minute or two until thickened. Slurries are generally added at the end of a cooking process. The coulis will thicken further upon cooling. For more information, enjoy this article about working with arrowroot.