Preheat oven to 400ºF/205ºC. Separate skin from the duck breasts. Lay the skins along the bottom of the terrine mold/loaf tin.
Add the ducks breasts, giblets and pork to a food processor and pulse until mostly broken down and mixed, leave some of the pieces of meat intact.
Using a mortar and pestle, grind the peppercorns, mustard seeds and cloves together.
Mix together the meat mixture, spices, garlic, shallots, parsley, egg, panko crumbs, salt, duck fat, and cognac. Fold in the hazelnuts and cherry preserves. Spoon this mixture over the skins in the terrine mold, press it down and smooth out. Cover with foil, place in a water bath in the oven and bake for 1 hour.
Turn off oven, but leave terrine in the oven for 2 hours without opening the door. Remove from the oven and allow to cool completely. Slice the terrine and serve with crusty bread. Remaining terrine can be re-covered and stored in the fridge.