Bring a large pot of water to a boil and add some salt. Cook the spaghetti according to package directions until just tender. Drain and reserve some of the cooking liquid.
Bring another skillet to medium heat and add 2 tablespoons olive oil in the skillet.
When the oil is hot, add in the Panko and stir frequently until golden and fragrant (3-4 minutes). Remove to a plate.
Add the last 2 tablespoons of oil to the pan and add the chopped onion. Sprinkle with salt and pepper and stir until softened.
Turn the heat on the onions to medium high and add in the zest of 1 lemon, juice of 1 lemon, capers, and thoroughly drained sardines.
Cook, stirring occasionally, until heated through. (About 2 minutes)
Add the drained pasta to the sardine mixture and toss well to combine. Add 1/2 cup chopped parsley and some of the reserved pasta cooking water (if needeto moisten. Stir in the bread crumbs and parmesan cheese.
Garnish individual plates with a wedge of lemon and more parsley if desired.