Method:

1. Cut the tops and bottoms off the onions then put them in a large bowl and cover with boiling water. Leave for 3–4 minutes, then drain. When cool enough to handle, peel the onions. After peeling, cut a small cross into the root end to stop the centres popping out during cooking.
2. Put the vinegars, sugar, salt, pared orange peel, fennel seeds, peppercorns and oil in a large saucepan with 350ml water. Place over a medium heat and stir until the sugar is dissolved.
3. Bring the liquid in the pan to a simmer, then add the onions and cook for 6–7 minutes, or until they are tender but still hold their shape. Remove from the heat.
4. Using a slotted spoon, pack the onions into the prepared jars and tuck the orange zest and the rosemary or thyme down the side of the jars.
5. Return the pan of liquid to the heat and bring to a vigorous boil for a few minutes. Pour the liquid over the onions and fill the jars to the top. Seal immediately with vinegar-proof lids. Turn the jars upside-down for a minute or so before putting them the right-way up and leaving to cool.
6. Store in a cool, dark place for up to a year. Once opened, keep in the fridge and use within 8 weeks.