For the sauce, heat the oil, bay leaf, rosemary, juniper berries, peppercorns and garlic in a deep, heavy-based, non-reactive saucepan over a very gentle heat. As the oil heats, increase the heat a little to allow the flavours to infuse the oil.
When the herbs start to sizzle, add the onion and season, to taste, with salt and freshly ground black pepper. Cook very gently without frying fiercely or letting the onions brown, for 15-20 minutes, or until the onion slices have softened and are covered by the oil.
Meanwhile, season the sardine fillets with salt and freshly ground black pepper. Sprinkle the flour onto a plate and dredge each sardine fillet in the flour until they are all coated on both sides.
Heat the vegetable oil in a deep-sided, heavy-based pan until a breadcrumb sizzles and turns golden-brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Alternatively, heat the oil in a deep-fat fryer.
Gently lower the sardines into the hot oil in batches and fry for one minute. The sardines shouldn’t colour or cook through. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Repeat the process with the remaining batches of sardines.
Increase the heat of the pan containing the onions and gradually add the vinegar to the pan in a thin stream. Remove the pan from the heat. Drain the sultanas then stir them into the onion mixture.
Spoon half of the warm onion mixture in a thin layer at the bottom of a deep ovenproof dish. Arrange the half-cooked sardines in a single layer on top, then cover with the remaining onion mixture. Cover the dish with cling film and set aside to cool and marinate. This should take 2-3 hours (the marinade will cook the fish). If marinating the sardines overnight, put them in the fridge and remove half an hour to bring them back to room temperature before continuing.
Meanwhile, for the pasta, take the chilled pasta dough and cut into pieces if necessary (to make it more manageable). Flatten each piece of pasta, then roll to a thickness of about 0.5cm/¼in using a rolling pin.
Pass one piece of pasta dough through a pasta machine set to its widest setting. Fold each edge of the rolled pasta dough over the middle section of the dough (so that it ends up one-third of its original width and three times its original height) and re-roll through the same setting.
Repeat this process without changing the setting of the pasta machine until the pasta is shiny and rectangular in shape. Then repeat the process with the remaining piece(s) of pasta dough.
Decrease the roller setting of the pasta machine by one grade and pass the piece(s) of pasta dough through the rollers once more. Each time you pass the pasta through the rollers, decrease the setting by one grade until you reach the penultimate or final setting and the pasta dough is smooth and supple.
Using a sharp knife, cut away any ragged edges from the pasta sheets, to create several sheets that are 20cm/8in in length.
If your pasta machine has a tagliatelle cutter, feed the pasta sheets through the machine one at a time, draping the strands of tagliatelle over a pasta drier or similar until needed. Repeat the process using the remaining pasta strips. Alternatively, cut the tagliatelle by hand using a sharp knife.
When the sardines have marinated, boil a large pan of salted water in preparation for the pasta.
Remove the garlic, herbs, peppercorns and berries from the marinade mixture.
Using a slotted spoon, transfer the sardine-and-onion mixture to a large, heavy-based pan. Heat over a low heat to warm the mixture through, stirring regularly and gently breaking the sardines into pieces as you do so.
Boil the tagliatelle in the salted, boiling water for 2-3 minutes, or until al dente. Drain, reserving a little of the cooking water.
Stir the tagliatelle into the sardine-and-onion mixture, stirring to coat the pasta in the sauce. Add a little of the reserved cooking water to the mixture if necessary, to loosen the sauce.
Serve immediately.