Trim off most of the dark-green parts of the leeks, leaving the white and just an inch or two of the light-green part. Cut into circles and soak in a large bowl of water to remove dirt. Drain and pat dry with paper towel to ensure all the dirt is removed.
Heat 1 tablespoon each of butter and olive oil in a large saute pan over medium heat. Add the leeks, sprinkle with the thyme, salt and pepper, and cook for about 10 minutes, turning every now and then, until the leeks are softened but not colored. In a small bowl combine chicken stock, cream and egg and whisk to combine. In a large mixing bowl crumble in cornbread muffins, then add leeks, chicken stock, a splash of cream and egg. Season well with salt and pepper, then stir to combine well.
Using a sharp, thin knife, cut the breast parallel to the board, almost in half, just stopping short of the outside edge. Open out the 2 halves as if you were opening a book. Now you've got a large piece of meat that will cook evenly and is thin enough to roll.
Preheat oven to 375 degrees.
Sprinkle the cut surface of the turkey with salt and pepper and spoon stuffing on top. Smear evenly on top with a rubber spatula. Now, starting with the left side, roll the turkey over the filling and into a cylinder. Tie in 4 places with kitchen twine. Season with salt and pepper. (You can bake remaining stuffing in a gratin dish to serve as dressing.)
Put the turkey in the roasting pan (with the leg-thigh pieces and wings; see above recipe) and sear all over. Transfer the pan to the oven and roast for about 25-30 minutes (internal temperature should reach 160 degrees F). Continue as outlined in above recipe to make gravy with pan drippings.