Finely chop the coriander stalks and leaves. Add to a food processor and pulse to make a coarse paste. Add the garlic, spices and chilli. Pulse again until a coarse paste is formed.
Add a little olive oil to loosen the paste (it should be the consistency of mayonnaise). Season with salt and pepper, then add lemon juice, to taste.
Spread the mix liberally on two of the sardine fillets and place the other fillets on top to create a 'sandwich'. Season the sardine skin with sea salt.
Preheat the grill to hot.
Heat a little oil in an ovenproof frying pan until hot. Fry the sardine 'sandwiches' for 1-2 minutes and then place under the hot grill. Cook until the skin is crisp and the fish cooked through.