Bring a large pot of salted water to a boil. Cook fettuccini until al dente. Reserve 1 cup cooking water and drain noodles.
Melt butter in a small skillet over medium/medium-high heat. Add panko and a pinch of sea salt and toast, stirring often, until breadcrumbs are fragrant and golden, 2 - 4 minutes.
Working quickly, slice onions into long strips and toss with warm fettuccini. Toss with 2 tablespoons olive oil, Parmesan, capers, and parsley (reserve some parsley for plating). Add a splash of pasta cooking water to keep things loose. Fold in the sardines, and add lemon juice, sea salt, and red pepper flakes to taste.
Timing plays a big role in making sure the pasta is comes out warm and the ingredients meld, so make sure the pasta is ready just as the other elements are and work quickly to get everything to the table. If you’re up for it, warm plates help too.