Preheat the oven to 450 degrees F. Heat 1 tablespoon butter in a large skillet over medium-high heat until it melts and starts to brown; add the apples, reduce the heat to medium and cook, stirring, until slightly tender, about 4 minutes. Stir in the honey and vinegar, then transfer to a large bowl to cool.
Melt 2 tablespoons butter in the same skillet over medium-low heat. Add the bread cubes and toss to coat, then transfer to one side of a large rimmed baking sheet; add the sausage to the other side. Bake until the sausage is cooked through and the bread is toasted, 8 to 10 minutes. Add the sausage and bread to the bowl with the apples; add the sage, thyme and rosemary.
Heat another 1 tablespoon butter in the skillet over medium heat. Add half each of the leeks, celery and parsnips, and 2 tablespoons water; season with salt and pepper. Cook until the vegetables are translucent, about 8 minutes, then transfer to the bowl with the sausage-apple mixture. Add the remaining leeks, celery and parsnips, and 2 tablespoons water to the skillet, season with salt and pepper and cook about 8 minutes; transfer to the bowl and toss. Warm a spoonful of the stuffing in the skillet with a touch of butter and taste the seasoning; adjust as needed. (It's so important to taste your stuffing while it's hot to get a sense of the seasonings and flavors.) Add the chicken stock to the bowl and toss to moisten all of the ingredients.
Reduce the oven temperature to 350 degrees F. Brush a 9-by-13-inch baking dish with 1 tablespoon butter. Transfer the stuffing mixture to the dish and dot with the remaining 3 tablespoons butter. Cover with aluminum foil and bake 40 minutes, then uncover and continue baking until golden brown, about 40 more minutes.