Thinly slice the yellow onions. In a large skillet, heat olive oil over medium heat. Add onions and sauté for 30–40 minutes until golden brown, deglazing with broth if necessary.

Melt butter in the same skillet, sprinkle in flour to create a roux, whisking continuously for about 2 minutes. Gradually add heavy cream, whisking until thickened, about 3–5 minutes. Mix in sautéed onions, salt, pepper, parsley, and grated Parmesan.

In a large pot, bring salted water to a boil. Cook cavatappi macaroni until just shy of al dente. Drain and mix into the béchamel sauce, ensuring each piece is coated.

Transfer the pasta mixture into an oven-safe baking dish and spread evenly.

Preheat oven to 350°F (175°C). Scatter caramelized onions atop the pasta. Mix breadcrumbs, additional Parmesan, and oregano in a bowl, then distribute over the pasta.

Bake for 10–15 minutes until topping is golden brown and crisp.

Remove from the oven, let cool for a minute, and serve hot, garnished with fresh parsley.