Preheat oven to 180 C / Gas 4. Grease a 23cmx13cm loaf tin.
Place sweet potato on a microwave-safe plate and pierce skin 5 or 6 times. Cook in the microwave, rotating halfway through, until tender, 5 to 8 minutes. Cool until easily handled, about 5 minutes. Peel and mash in a small bowl.
Mix wholemeal flour, bicarbonate of soda, cinnamon, nutmeg and salt together in a large bowl.
Whisk apple sauce, eggs and maple syrup together in a separate bowl. Mix in mashed sweet potato.
Pour apple sauce mixture over flour mixture; stir to combine. Fold in walnuts. Pour into the prepared loaf tin.
Bake in the preheated oven until a skewer inserted into the centre comes out clean, 45 to 55 minutes. Cool in the tin for 15 minutes; turn loaf out onto a wire cooling rack and let cool completely, about 20 minutes.