Bring a large pot of water to a boil and salt it heavily. While the water comes to a boil, make the topping and start the sauce.

To prepare the topping, heat a large skillet over medium-high heat; add 2 tablespoons oil and breadcrumbs. Stirring frequently, sauté breadcrumbs until golden brown, about 5 minutes. Transfer to a bowl and mix thoroughly with lemon zest, juice, parsley and salt to taste. Set aside.

To prepare sauce, wipe out the skillet and heat 1/4 cup oil over medium-high heat. Add onions, garlic, celery, red pepper flakes and a pinch of salt. Sauté until vegetables start to soften, about 7 minutes. Add sardines, tomatoes, currants and walnuts. Cook, stirring occasionally, for another 5 minutes.

While the sauce cooks, add pasta to the boiling water and cook for about 3 minutes, or until it is not quite al dente. Drain through a colander, reserving 1/4 cup pasta water. Add cooked pasta and reserved cooking water to the skillet with the sauce, turning the heat down to medium. Toss gently until pasta has absorbed the sauce and is cooked to the al dente stage, 3 to 4 minutes more.

Serve pasta in bowls with the lemon-panko topping and a drizzle of olive oil.