Saute the sausage in a large heavy skillet over medium-high heat until cooked through, crumbling coarsely with the back of a spoon, about 10 minutes. Using a slotted spoon, transfer the sausage and drippings to a large bowl. Melt the butter in the same skillet over medium-high heat. Add the leeks, apples, celery and poultry seasoning to the skillet and saute until the leeks are soft, about 8 minutes. Mix in the drained cranberries, sage and rosemary. Add the mixture to the sausage, then mix in the croutons and parsley. Next add the chicken stock a little at a time until the stuffing is very moist. Be sure not to overdo it; it shouldn't be mushy. Season with salt and pepper. Place in a casserole dish. (The stuffing can be made to this point 2 days before Thanksgiving, refrigerated.)
Bake in a 14-inch oval or 9 by 13-inch rectangular casserole dish and place, uncovered, in the oven for 20 to 30 minutes, until the top is crispy and the center piping hot. Remove and serve immediately.
If stuffing a turkey, the stuffing must be cold before being placed in the bird.