Pulse bread in a food processor until coarse crumbs form. Heat 2 tablespoons oil in a large skillet over medium-high. Add crushed garlic, and cook until golden, 2 to 3 minutes. Add breadcrumbs and 1 tablespoon oil, and stir until well coated. Season with salt, and cook until breadcrumbs are crisp and golden brown. Place on a rimmed baking sheet lined with paper towels to drain. Discard garlic, and let breadcrumbs cool completely.
Heat 3 tablespoons oil in each of 2 large skillets over medium heat. Divide onions between skillets, and cook, stirring frequently, until caramelized and golden brown, 10 to 15 minutes. (Add a little more oil if necessary.) Combine cooked onions in 1 skillet. Add sliced garlic, and cook over medium heat until golden, about 3 minutes. Add anchovies and red-pepper flakes, and cook for 2 minutes.
Meanwhile, cook spaghetti according to package directions in salted water. Drain, reserving 1 cup cooking liquid. Transfer spaghetti to skillet, and toss well. Add reserved cooking liquid, parsley, and butter, and season with salt and pepper. Mix well, then top with breadcrumbs, and garnish with parsley.