In a large saute pan, heat oil over low heat. Add onion and garlic; cook and stir until onion is tender. Stir in capers, tomatoes, lemon juice, and parsley. Season with red pepper flakes to taste. Simmer gently for 3 minutes to thicken sauce. Fold in tuna, and heat through.
While sauce is cooking, add pasta to a large pot of rapidly boiling water; cook till just tender. Drain well.
