Preheat oven to 350 degrees and place hazelnuts on a rimmed baking sheet. Bake until lightly golden brown, about 10-15 minutes. Remove from oven and pour into a clean dish towel. Rub nuts with the towel until the skins peel off. Chop nuts in half and place in a large bowl. Set aside.
Heat a large skillet over a medium flame. Remove sausage from casing and break apart with a spatula. Cook sausage until brown and crumbly and pour into the bowl with the nuts.
Return pan to stove and add butter. When butter has melted add onions, celery and fennel and cook, stirring occasionally, until the vegetables are tender and translucent, about 10 minutes. Pour into the bowl with the sausage and nuts.
Add the parsley, chopped fennel frond, rosemary, thyme, salt, pepper and stuffing mix and toss to combine. Gently toss stuffing mixture with the stock and place in a well-greased covered casserole dish (2 quart) and refrigerate until ready to use.
Take stuffing from the refrigerator about 1 hour before you are ready to bake it. Preheat the oven to 400 degrees. Bake stuffing, covered, for 30 minutes. Remove cover from stuffing and bake 15 minutes more, until golden brown on top. Serve immediately.