(optional) In a 5 litre Dutch oven, heat the oil over medium-high heat. Sprinkle the beef with the salt and pepper. In batches, add the beef and cook, turning occasionally, until browned on all sides, 8 to 10 minutes. Using a slotted spoon, transfer the browned beef to a plate and set it aside. Add the onion and chile pepper to the Dutch oven. Reduce the heat to medium and cook, stirring often, until the onion has softened, about 3 minutes. Add the curry powder and coriander, and stir for 1 minute. Return the beef to the Dutch oven, and stir in the water. Reduce the heat to low and cook, covered, until the meat is tender and the liquid has almost completely evaporated, about 1 hour. (If the stew looks as if it's drying out, add a little more water.) Preheat the oven to 350. Lightly butter a 23 cm square baking dish. In a large bowl, combine the plantains and spinach, and mash together with a fork until fairly smooth. Stir in the beef stew. Spread the mixture evenly in the prepared baking dish, and dot the top with the butter. Bake until the top is golden brown, about 30 minutes. Sprinkle with the grated coconut if desired, and serve immediately.