Fill a large pot with water and set it over high heat to boil.

Set a large skillet over medium heat and add 2 tablespoons of the olive oil.

Add the panko breadcrumbs to the skillet and cook, stirring often, for 3 to 4 minutes, until they are evenly golden and crisp.

Transfer the toasted breadcrumbs to a small bowl and stir in a small pinch of the kosher salt. Set the bowl aside for serving.

When the water reaches a full boil, add enough salt to make it taste lightly seasoned, then add the spaghetti to the pot.

Stir the spaghetti for the first 30 seconds so it does not stick together, then cook it according to the package directions until it is tender but still a little firm in the center.

While the pasta cooks, return the skillet to medium heat and add the remaining 2 tablespoons olive oil.

Add the chopped onion and the red pepper flakes to the skillet. Cook for 6 to 8 minutes, stirring now and then, until the onion is very soft and lightly golden.

Ladle 1/2 cup of the hot pasta water into the skillet. Add the golden raisins and let the liquid bubble gently for about 2 minutes, until the raisins plump up.

Add the drained sardines to the skillet. Use a wooden spoon to break them into small bite-size pieces, then cook for 1 to 2 minutes, just until warmed through.

Scoop out 1 cup of the pasta cooking water and keep it beside the stove.

Drain the spaghetti in a colander as soon as it is ready.