Prepare Your Onions: Thinly slice the onions into long strips. Heat a large skillet over medium heat, add olive oil, and sauté the onions for about 30–40 minutes, stirring occasionally.
Caramelize the Onions: Let the onions develop their natural sweetness through slow caramelization. If they stick, deglaze with beef broth and simmer for an additional 10 minutes.
Create Béchamel Sauce: In the same skillet, reduce heat slightly, melt butter or use olive oil, whisk in flour, and gradually incorporate heavy cream until thickened.
Combine Ingredients: Stir in two-thirds of the caramelized onions into your béchamel sauce along with seasonings and grated Parmesan cheese.
Cook Your Pasta: Bring a large pot of salted water to a boil, add cavatappi macaroni, and cook until just shy of al dente.
Assemble and Bake: Mix the cooked pasta with the béchamel mixture, transfer to a baking dish, top with remaining onions and breadcrumbs, and bake at 350°F for 10–15 minutes.