This is a matter of preference - the grated raw vegetables cook perfectly will in the time it takes the meatloaf to cook, and I like the bits of vegetables to be visible. If you are really trying to hide the vegetables in the meatloaf, you can certainly saute them in a little sunflower oil and allow them to cool before adding to the meatloaf. Softening the vegetables in oil will certainly help to make them less visible in the finished dish.