Heat six tablespoons of the oil in a large frying pan over a low to medium heat. Add the fennel, onions, fennel seeds and chillies, then half-cover the pan with a lid. Fry gently for 18-20 minutes, or until the vegetables have softened but not coloured.
Add half of the sardines and all of the raisins and pine nuts and stir until well combined. Remove the lid from the pan, then continue to cook over a low heat for a further 8-10 minutes, stirring and crushing the ingredients with a wooden spoon every now and then, until the sardines have fallen apart and the mixture is well combined.
Increase the heat, add the white wine and water and bring the mixture to a simmer, stirring well. Continue to simmer until the volume of liquid has reduced by half, then return the temperature to a low heat and simmer the sauce very slowly for a further 4-5 minutes, or until it has thickened. Season, to taste, with salt and freshly ground black pepper.
Arrange the remaining sardine fillets, skin-side up, on top of the sarde sauce, side by side, then drizzle over two tablespoons of the extra virgin olive oil.
Cover the pan again with the lid and cook the sardines and sauce over a low heat for 7-10 minutes, or until the sardine fillets are cooked through. (NB: There is no need to stir the mixture at this stage.)
Meanwhile, cook the linguine according to packet instructions in a large pan of boiling, salted water. When the pasta is al dente, drain well.
Add the sarde sauce, cooked whole sardine fillets and the remaining two tablespoons of olive oil to the pan with the cooked pasta in it and mix well until the linguine is coated in the sauce.
To serve, divide the pasta con sarde equally among four serving plates. Squeeze a little lemon juice, to taste, over each serving. Garnish with the lemon zest and reserved fennel fronds.
