This stuffing is a fun variation on a classic -- sausage, leeks and cornbread add moisture and flavor, while cubes of artisan bread and plenty of sage and celery please the traditionalists in the family. It can be made in advance and kept refrigerated until you are ready to bake it or stuff the turkey, though I prefer to cook my stuffing in a casserole dish, which gives it a firmer texture and helps the bird cook faster.
Heat a large skillet over medium heat. Add the breakfast sausage, breaking it apart with a wooden spoon, and cook, stirring occasionally, until no longer pink. Remove from pan with a slotted spoon and place in a large mixing bowl.
To the rendered fat add the butter then the onion and celery, and sauté, adjusting the temperature if necessary so they don't brown. Cook for about 15 minutes, or until soft and translucent.
Add the leeks and cook for five minutes more. Set aside to cool.
When onion mixture has cooled, pour it out, with all the accumulated fat, into the large bowl with the sausage. Add the bread cubes, cornbread cubes, herbs and parsley. Season with salt and pepper. Toss gently, so the cornbread doesn't get too crumbly, and taste for seasonings. Place in a large, buttered (3-3 1/2 quart) casserole dish, cover and cool completely. This stuffing can be assembled and refrigerated one day ahead.
Preheat the oven to 325 degrees. Bring stuffing to room temperature. Pour over turkey stock and toss gently to coat. Cover and bake for 20 minutes, uncover and bake for 10 minutes more.