Thinly slice the onions and sauté in olive oil for 30-40 minutes until caramelized.
Add beef broth to the onions if they stick, simmer for 10 minutes, then set aside.
Melt butter, whisk in flour, then gradually add heavy cream to thicken.
Stir in two-thirds of caramelized onions, seasonings, and Parmesan cheese.
Cook cavatappi macaroni in salted water until just shy of al dente, then combine with béchamel.
Transfer to a baking dish, top with remaining onions and breadcrumbs, and bake at 350°F until golden brown.