Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish. Pour the stuffing mix into a large bowl and set aside.
4 tablespoons of the butter in a large skillet over medium heat. Add the celery and leeks, sprinkle with salt and pepper and cook until very soft, 10 minutes. Stir in the mustard and lemon juice
and cook until the liquid is absorbed, 3 minutes. Remove from the heat.
Add the leek and celery
, parsley, sage and eggs to the stuffing
mix. Pour the warmed stock over, toss to combine and then spoon into the baking dish
the top with the remaining 4 tablespoons of butter and bake until lightly browned on top, 30 minutes. Serve warm.