Thinly slice the onions. In a large skillet, heat olive oil over medium heat. Add onions and a pinch of salt, sauté for 30–40 minutes, stirring occasionally until golden-brown.

In the same skillet, melt butter over medium-low heat. Gradually whisk in flour, cooking for about 1 minute. Slowly pour in heavy cream while whisking until sauce thickens, about 5 minutes. Stir in two-thirds of caramelized onions, salt, pepper, oregano, and Parmesan cheese.

In a large pot, bring salted water to a boil. Add cavatappi macaroni and cook until just under al dente, about 7–8 minutes. Drain pasta, reserving a cup of pasta water, and return to pot.

Preheat the oven to 350°F (175°C). Pour béchamel sauce with onions over drained pasta and mix gently. Transfer mixture to a greased baking dish, spreading evenly. Top with remaining caramelized onions and breadcrumbs.

Bake for 10-15 minutes until the top is golden brown and the edges are bubbling.

Remove from oven and let cool for a few minutes. Garnish with fresh parsley and serve warm.