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.