Heat the chilli, bay leaf coriander seeds, vinegar and verjuice together in a pan until hot. Set aside.
Brush the whiting with a little olive oil and pan-fry for 2 - 3 minutes each side, depending on their size. Place the fillets in a baking dish in a single layer with the sliced onion and pour over the marinade - leave to cool and serve at room temperature.
Note: The red layer of the Spanish onion contains a powerful phytonutrient called anthocyanin.