Infuse olive oil with bay leaf, rosemary, juniper, peppercorns, and garlic in a saucepan over low heat.
Add sliced onions and seasoning, then cook gently for 15-20 minutes until softened but not browned.
Season sardine fillets and dredge them in flour until coated on both sides.
Heat vegetable oil in a deep pan or fryer until a breadcrumb sizzles.
Fry sardines in batches for one minute without browning, then drain on kitchen paper.
Add vinegar to the onion pan, remove from heat, and stir in the drained sultanas.
Layer sardines and onion mixture in a dish and marinate for 2-3 hours to cook the fish.
Roll out chilled pasta dough to a thickness of 0.5cm.
Pass dough through a pasta machine on the widest setting, folding and re-rolling several times.
Continue rolling while decreasing settings until the pasta is smooth and thin.
Cut pasta sheets into 20cm lengths.
Cut sheets into tagliatelle strands using a machine cutter or a sharp knife.
Remove whole spices and garlic from the marinated sardine mixture.
Warm the sardine and onion mixture in a pan, gently breaking the fish into pieces.
Boil tagliatelle in salted water for 2-3 minutes until al dente.
Drain pasta and toss with the sauce, adding reserved cooking water to loosen if needed.
Serve the dish immediately.