To clean the sardines, first scrape and rub off the scales. To remove the head and innards, slice just behind the gills, severing the head from the back (dorsal side) of the fish, leaving it attached at the belly. Pull the head away from the body, drawing the attached innards from the fish. Slice the belly to open the cavity and scrape out any remaining guts. Rinse the sardines well, washing away the scales, and pat dry with paper towels.

Salt the sardines lightly all over and dredge well in flour.    Pour vegetable oil into the skillet to a depth of 1/8-inch or so and set over medium-high heat until it sizzles on contact with fish. Shake excess flour off the sardines and lay in as many as will fit in the pan with a bit of space between them. Fry on the first side for several minutes until the skin is crisp and golden and the flesh is cooked, then carefully flip them over and fry on the second side until evenly browned and just cooked through.

Lift the fish from the oil with a slotted spatula and set on paper towels. Salt the fish again lightly. Fry, drain and season the remaining sardines, in the same way.

When the fish are all cooked, pour out and discard the vegetable oil and wipe out the pan (or use another large skillet if you have one). Pour in the ½ cup olive oil and set over medium heat. Stir in the sliced onions, rosemary and bay leaves and season with 1 teaspoon salt. Cook slowly, stirring occasionally, until the onions are soft and translucent, 5 minutes or so.

Stir together the red wine vinegar and water and pour into the pan. Raise the heat and cook the onions in the boiling liquid, stirring now and then, until it is reduced by half. Pour in the wine, bring to a boil, then adjust the heat to maintain steady bubbling. Stir in the coarsely ground black pepper and cook until half of the wine has been absorbed or evaporated. Stir the onions so they cook evenly and just begin to color but don’t let them overcook or caramelize as they should still have some texture to the bite.

Remove the skillet from the heat and assemble the dish while the saor is hot. Spread a spoonful of onions in the bottom of the casserole dish and lay half the sardines on top in a single layer. Cover with a denser layer of onions, then with another layer of sardines. Spread the rest of the onions on top—reserve enough to make this a generous layer—then scrape in all the juices from the skillet. Drop the rosemary and bay leaves on top.

Let the dish cool to room temperature and marinate for at least 2 hours before serving. For the best flavor, cover the dish with plastic wrap and marinate for 1 to 3 days in the refrigerator. Turn the layers occasionally to distribute the saor; always leave a layer of onions on top. Remove the dish—or a portion of sardines and saor—from the refrigerator several hours ahead, to warm to room temperature. Discard the herbs before serving.