Cook spaghetti as per the directions on the package; set aside.
In the meantime, heat oil in a large skillet over medium heat. Add the onions, garlic, and a pinch of salt, and simmer until tender and fragrant—about 3 minutes.
Add the drained sardines to the skillet and flake them into chunks with a spatula or spoon; stir to mix.
Once the sardines are just warmed through, which should only take about 2 minutes, transfer the cooked pasta to the skillet; toss to coat. Add the lemon juice, lemon zest, parmesan, arugula, and a generous amount of cracked black pepper. Toss again to even distribute the ingredients. Taste, and add salt as needed.
Finally, toss in the toasted breadcrumbs. Serve immediately.