Combine meat, fat, bones, salt, sugar, and bay leaves in a bowl.
Grind juniper, peppercorns, cloves, and nutmeg into powder and mix with meat.
Cover and refrigerate the mixture overnight.
Preheat oven to 140°C.
Transfer meat to a pan with wine, thyme, garlic, and duck fat.
Heat until near boiling, cover with pork skin and a lid, then bake for 2.5 to 3 hours.
Remove from oven and let cool for 15 minutes.
Discard skin, bones, and bay leaves, then strain and reserve the fat.
Blend meat and remaining liquid, gradually adding fat back until soft.
Season with extra ground spices and set in a cling-film-lined terrine mould in the fridge.