01 - Set oven to 350°F and allow to fully preheat.
02 - Using a food processor, pulse baguette into coarse crumbs. Toss breadcrumbs with olive oil, salt, black pepper, and oregano. Spread evenly on a baking sheet and toast in the oven for 8 to 10 minutes, or until golden brown. Set aside.
03 - Heat a large skillet over medium heat and add a small amount of olive oil. Add sliced onions and sauté slowly for 30 to 40 minutes, stirring occasionally, until a deep golden brown is achieved. Add sugar to accelerate browning if desired. If onions stick, pour in beef broth and simmer for 10 minutes until liquid is absorbed. Remove onions from skillet and set aside.
04 - In the same skillet, melt butter over medium-low heat. Whisk in all-purpose flour and cook for 1 minute. Gradually add heavy cream, whisking constantly to avoid lumps. Simmer for 3 to 5 minutes, or until thickened. If mixture thickens unevenly, whisk in cold water, 1 tablespoon at a time, until smooth.
05 - Fold in two thirds of the caramelized onions, Parmesan cheese, garlic powder, smoked paprika, ground nutmeg, 1 teaspoon kosher salt, and a portion of chopped parsley. Simmer the mixture for an additional 2 to 3 minutes.
06 - Bring a large pot of salted water to a boil. Cook pasta, stirring occasionally, until just under al dente, about 2 minutes less than package directions. Drain thoroughly.
07 - Mix drained pasta with béchamel-onion sauce in a large bowl or skillet, ensuring the pasta is evenly coated.
08 - Transfer the mixture to an oven-safe skillet or baking dish. Evenly distribute the remaining caramelized onions over the top and sprinkle with the prepared breadcrumbs.
09 - Bake uncovered at 350°F for 10 to 15 minutes, until the surface is golden and crisp.
10 - Remove from oven, garnish with additional chopped parsley, and serve immediately while hot.