Method:
Cook pasta in a 6-quart pot of boiling salted water until al dente.
Meanwhile, heat oil in a large skillet over high heat until hot but not smoking. Add onions, garlic, salt, pepper and red pepper flakes and cook, stirring occasionally, until golden, 6 to 8 minutes. Add walnuts and cook for 2 to 3 minutes, until lightly toasted. Add tomatoes, breaking them up with a spoon, and simmer for 5 minutes. Stir in tuna and raisins.
When pasta is done, drain well and return to pot. Add tuna mixture to pasta in pot, toss gently and warm over low heat for about 30 seconds. Garnish with parsley and serve immediately.