Melt the butter in a tagine or large, heavy saucepan over medium heat. When hot, add the garlic and fry for a few minutes, until golden. Add your spices and fry for another minute or so, until aromatic.
Add your nectarine, samphire and onions and mix well to coat in the spices. Cook for 5 minutes, then add the pomegranate syrup and nut butter. Mix well.
Now take your fish and lay it in the middle of your saucepan (cut to fit snugly in the pan if necessary). Add enough stock to cover most of the fish, then lower the heat, cover with a lid and simmer gently for around an hour, stirring every 15 minutes or so. Add more stock if it begins to dry up.
After the hour, check the gravy. If it's still very liquidy, carefully remove the fish and keep warm, then add a tablespoon of tapioca starch to the pan and cook, stirring constantly, until it thickens.
Once thickened, add the EVOO, coriander and salt and pepper to taste. Mix well, then pour the sauce over the fish. Serve immediately, with bulgur wheat or couscous. And a dollop of yoghurt, if you like.