Bring a large pot of salted water to a boil.

Melt butter in a skillet over low heat.

Add diced onions, salt, and cover. Cook for 5 minutes.

Add sugar to onions and stir frequently for 10-15 minutes until dark in color.

Cook spaghetti according to package directions for al dente.

Add a tablespoon of water to onions and scrape up brown bits. Cook 5 more minutes. Set aside.

Tear day-old bread into smaller pieces.

Pulse bread in a blender/food processor until crumbs form.

Remove onions from skillet. Heat half of the olive oil over low heat.

Add breadcrumbs and stir to coat. Toast until golden brown and season with salt. Remove and set aside.

Reserve some pasta cooking water.

Drain pasta and set aside.

Add remaining olive oil to the skillet. Heat over low heat.

Add garlic, red pepper flakes, and anchovies.

Smash anchovies into the oil. Cook for 2 minutes. Turn off heat.

Strain the oil to remove garlic and anchovy pieces. Set the oil aside.

In a medium bowl, mix pasta with breadcrumbs, caramelized onions, garlic and anchovy oil, parsley, and reserved pasta water. Adjust seasoning as needed.

Sprinkle a little kosher salt on top before serving.