While you wait, add the yellow onion, capers, olives, anchovies/sardines, and red pepper flakes to a food processor. Process until smooth.
Heat the olive oil in a pan over medium to medium-high heat. Add the contents of the food processor and cook 8-10 minutes, stirring regularly. Add the crushed tomatoes and tomato paste. Bring to boil and simmer until your pasta is cooked.
Add your pasta to the boiling water. Cook until al dente. Drain and add to the sauce. Toss to coat.