Thinly slice the onions into long strips.
Heat a large skillet over medium heat and add olive oil. Sauté the onions for about 30–40 minutes, stirring occasionally.
Pour in beef broth and let it simmer for an additional ten minutes.
In the same skillet, melt butter over low-medium heat. Whisk in flour and gradually add heavy cream.
Stir in two-thirds of your caramelized onions into the sauce and let it simmer. Season with salt, pepper, chopped parsley, and grated Parmesan cheese.
Bring a large pot of salted water to boil. Cook your cavatappi macaroni until just shy of al dente.
Combine your cooked pasta with the béchamel sauce until well mixed.
Transfer everything into a baking dish! Top with remaining caramelized onions and sprinkle with breadcrumbs.
Bake uncovered at 350°F (175°C) for about 10–15 minutes until golden crust forms on top.
Serve immediately, garnished with fresh parsley if desired.