In a 12 to 14-inch saute pan place 3 tablespoons oil and onions and place over medium heat. Cook slowly until medium golden brown, about 20 to 25 minutes, and then set aside.
Bring 6 quarts water to boil and add 2 tablespoons salt.
Clean and gut anchovies and remove fillets. In another 12 to 14-inch saute pan, heat remaining oil until smoking. Add anchovies, chilies, and garlic and cook until fish has broken down, about 4 to 5 minutes. Add onions to anchovy mixture and simmer.
Drop spaghetti into boiling water and cook according to package instructions until al dente. Drain and toss into pan with anchovy mixture. Add parsley and stir to coat. Pour into a warmed serving bowl, sprinkle with bread crumbs and serve.
Recipe courtesy of Mario Batali