Using clean hands (or a clean tea towel/dish towel) squeeze out any excess liquid from the grated carrot and zucchini. Add them to the bowl along with the parsley, mustard, meat, panko breadcrumbs and egg. Season with salt and pepper (if liked). Using your hands, combine the mixture, mixing for a couple of minutes. Form the mixture into golf ball–sized meatballs. If you have time, chill them in the refrigerator for 10 minutes, but this is not vital.