Preheat the oven to 350 degrees. \nPlace the turkey neck on a baking sheet and roast until browned, about 15-20 minutes.\nMeanwhile, add the gizzard to a large saucepan and sauté until browned. Remove the gizzard and add the onions, fennels, carrots and Hong Kong sauce to the pan. Sauté until the vegetables have softened, about 8-10 minutes. Stir in tomato paste until vegetables are coated. Once coated, add the wine and reduce. Once most of the wine has reduced, add chicken stock and turkey neck and gizzard. Bring to a boil, then lower the heat to a simmer. Continue cooking over a gentle simmer until the liquid has reduced by half.\nStrain the stock and return to the stove top. Add in the croissant pieces and cook until they have absorbed some of the liquid, about 2-3 minutes. \nTransfer the mixture to a blender, or use an immersion blender, and buzz the mixture until smooth and creamy. If you desire a thicker gravy, return the gravy to the stove and continue cooking until reduced to desired consistency. Season to taste with salt and serve warm.