1. Peel the onion. Slice it along with the fennel bulb.
2. Place the vinegar in a pot along with the water and sugar. Bring to the boil and add the herbs, spices and vegetables. Simmer for 5 mins and turn off. Heat a non-stick pan to high.
3. Add the some oil and sear the sardines fillets skin side down.
4. Lay the sardines in a shallow dish. Once the vinegar mix has cooled pour over the sardines.
5. Leave to pickle for a minimum of 4 hours but over night is best.