Bring 4 quarts water to boil in large pot. Meanwhile, process bread in food processor (or food chopper) until coarsely ground. Heat 2 tablespoons oil in large skillet over medium-high heat until shimmering. Add bread crumbs and cook, stirring occasionally, until light golden, about 3 minutes. Add half of garlic and 1/4 teaspoon red pepper flakes and cook until fragrant, about 30 seconds. Transfer to bowl and season with salt (may be omitted).
2
Wipe out skillet and add remaining 4 tablespoons oil, remaining half of garlic, and remaining 1/2 teaspoon red pepper flakes. Cook over medium-low heat until garlic is just golden, 1 to 3 minutes. Remove pan from heat.
3
Add 1 tablespoon salt and spaghetti to boiling water. Cook until al dente. Reserve 1/4 cup cooking water, drain pasta, and return to pot along with reserved water. Stir in oil mixture, Parmesan and parsley. Season with salt and sprinkle with bread crumbs. Serve.
