Adjust oven rack to middle position and preheat oven to 450°F. In large Dutch oven, melt 5 tablespoons butter over medium high heat until foaming subsides (don't allow butter to brown), about 2 minutes. Add sausage and mash with stiff whisk or potato masher to break up into fine pieces (largest pieces should be no greater than 1/4-inch). Cook, stirring frequently until only a few bits of pink remain, about 8 minutes. Add onions, celery, garlic, and sage and cook, stirring frequently, until vegetables are softened, about 10 minutes. Remove from heat and add half of chicken stock.

Using poultry shears, cut off and remove any back portion that may be attached to the turkey (there may not be any). Fill cavity under turkey breast and under flap of fat around neck with stuffing. Transfer remaining stuffing to a buttered 9 x 13-inch casserole dish and place turkey on top.

Using your hands, carefully separate the turkey skin from the meat by inserting at the bottom of the breast, being careful not to tear it. In a small bowl, combine remaining butter with remaining parsley and oregano. Add 1 tablespoon kosher salt and 1/2 teaspoon freshly ground black pepper. Stir with a fork until homogenous. Rub mixture evenly over and under turkey skin.

Transfer to oven and roast until stuffing starts to brown, about 45 minutes. Remove from oven, transfer turkey to a wire rack set in a foil-lined rimmed baking sheet, and return turkey to oven. Continue roasting until turkey is golden brown and crisp, and thickest part near bone registers 145 to 150°F on an instant read thermometer, about 30 minutes longer. Remove from oven, transfer to a plate, and let rest for 20 minutes.

Meanwhile, pour exuded juices back over stuffing. Return stuffing to oven and cook until it's golden brown and registers 160°F on an instant read thermometer, about 15 minutes. Carve turkey, spread over stuffing, and serve.