clean the sardines
Clean the sardines. Using a small knife, gently rub the skin of the sardine until the scales are removed. Then cut open the belly of the fish using the same small knife. Run your thumb along the spine and push out the bloody guts. Rinse the sardines under cold running water. Drain well on a kitchen towel and tap dry.
fry the sardines
Add flour to a plate. Coat the sardines in flour and place on a large plate. Pour both oils into a large pan and place over high heat. Bring the oil to 170 °C / 340°F. Working in batches of two, fry the first batch of sardines. Fry for 1 - 2 minutes per side or until golden but not brown. Transfer to a plate lined with paper towels and repeat the process with the rest of the sardines. Set aside until needed.
Add olive oil to a pan. Cut the onion into thick slices and cut the garlic into thin slices. Add the onion to a pan and saute for 2 - 3 minutes for the onion to soften. Add the garlic and halved cherry tomatoes, if using. Fry for 2 minutes, then increase the heat and pour the white wine, vinegar, and water. Bring to a boil, then add the bay leaf, rosemary, peppercorns, and breadcrumbs. Stir to combine and cook for 6 - 8 minutes or until the alcohol evaporates. Remove from the heat. Stir half the chopped parsley, season with salt, stir, and set aside.
Choose a small baking dish. Arrange half the sardines in a single layer, then pour half of the marinade over the fish. Arrange the rest of the sardines on top and cover with the rest of the marinade. Add lemon slices and sprinkle with chopped parsley. Bring to room temperature, then cover with clingfilm and place in the fridge for at least 3 - 4 hours or overnight.
Serve the Marinated Sardines as an appetizer or main dish. Offer fresh bread and a glass of good white wine as a side.