Prehaet the oven to 190 C/375 F. Grease an ovenproof dish with butter. Melt half the remaining butter in a frying pan, add the onion and cook over a low heat until softened. Meanwhile, remove any stringy threads from the celery and cut it into 2.5cm pieces. Peel the sweet potatoes and cut into slices about 2mm thick.
Mix together the mace, allspice and caraway seeds. Arrange one-third of the sweet potato in a layer in the base of the prepared dish and sprinkle with some of the spice mixture and a little of the garlic. Season with salt and pepper, then add half the celery and top with the onion. Repeat the layers with half the remaining sweet potato and the remaining spice mixture and celery. Arrange the remaining sweet potato in an overlapping layer on top.
Pour in the cider or apple juice. Dot the top with the remaining butter. Cover with buttered greasproof paper and bake in the preheated oven to cook until the sweet potatoes are soft and slightly browned. Serve hot, with yoghurt.

ENJOY!