Peel and quarter the onion, push it through the fine slicing attachment of your food processor, and tip into a large pan on a medium heat with the butter. Pick, finely slice and add the sage leaves, then fry for 4 minutes, or until soft, stirring occasionally. Meanwhile, wash and trim the Brussels sprouts, clicking off any tatty outer leaves. In batches, push them through the same fine slicer. Add them to the pan, turn the heat up, cover, and fry for 10 minutes, or until soft, adding a splash of water, if needed. Toss in the Worcestershire sauce, then turn the heat off and gun in the unpeeled garlic through a garlic crusher. Stir well, season to perfection and serve.