Fill large pot with water and bring to boil; add 2 teaspoons of the olive oil and two teaspoons of salt.
Add pasta to water and cook until al dente, about 7 mins. Make sure to reserve at least one cup of the pasta water before straining.
While pasta is boiling, heat one tablespoon of the olive oil in a heavy-bottom fry pan on medium-low heat; toss in red pepper flakes and walnuts.
Add onions to the pan and sauté for about five minutes until tender. Add the remaining tablespoon of olive oil, then the garlic, and cook for 30 seconds to one minute, making sure not to burn. Increase heat to medium high and immediately add the sardines and capers; continue to sauté for another few minutes. Optional: add white wine.
Add the pasta to the pan with the sardines and toss gently, making sure to incorporate all ingredients with the pasta. When the pasta is hot and the pan is getting dry, add 1/2 cup of the pasta water, making sure to loosen any bits at the bottom of the pan.
Place pasta in large serving dish and sprinkle with bread crumbs, pecorino, parsley and more red pepper flakes if desired.
Serve steaming hot!