My gravy is always delicious, if I say so myself. When the turkey is cooked scatter plain flour into the juices (if there is a lot of fat, skim some off first) and stir over heat for a few minutes till it goes brown. you can tell when it's cooked because it starts to feel a bit "sandy". Add water that the veg have been cooked in and continue stirring, with a whisk preferably. Check seasoning and cook till the gravy is as thick as you like it. Don't worry if its lumpy. What do you think sieves were invented for?