To thicken the sauce for this dish, you add a cornstarch slurry at the end. You make the slurry by mixing 1 tablespoon cornstarch with 1 tablespoon cold water together in a small bowl. Then stir or whisk it into the sauce and simmer until thickened.  If you're wondering why you don't add the cornstarch directly to the sauce, it's because it will clump up if you add it to hot liquid.  That's why you typically add cornstarch to cold liquid and then stir it into sauces to thicken them.