Preheat the oven to 400 F/ 200 C/ gas mark 6. You will need a large baking tray.
Cut the sweet potatoes in half lengthways, place on the baking sheet skin side down and brush the open side with the oil. Bake for 35-40 minutes (they need to be tender).
While the potatoes are in the oven cook the fish in a bit of water (do not overcook it). Strain thoroughly and combine with the spinach (while still hot).
Add the coconut milk, lime juice and zest and seasoning. Stir and set aside.
Remove the sweet potatoes from the oven, leave until cool enough to handle, then scoop out the middle (using a melon baller or large spoon) leaving a few millimeters around the skin (at this point switch the oven back on, gas mark 7) and add to the fish mixture. Stir well, taste to check if more seasoning is needed and spoon the mixture into the potato skins. Sprinkle with the desiccated coconut, drizzle with a little oil and bake for 8 minutes (alternatively, pop under a grill for 4-5 minutes). Serve immediately.