1. First, peel the onions and cut them into rings.
2. Wash and roughly chop the sage.
3. Then put the onion rings, the sage and the sugar in a jar.
4. Seal the jar properly and leave to sit for around eight hours at room temperature until the sugar dissolves into syrup.
5. After three days, remove the sage and onions and keep the syrup in the fridge for up to six months.
6. Tip: A teaspoon of syrup is particularly good for your throat.