Preheat the oven to 350 degrees F (175 degrees C).
Heat butter over medium-low heat in a Dutch oven or a heavy-bottomed saucepan. Add onion and kosher salt; cook, stirring occasionally until softened and lightly golden, about 20 minutes.
Meanwhile, bring a large pot of lightly salted water to a boil. Cook cavatappi in the boiling water, stirring occasionally, until tender yet firm to the bite, about 9 minutes. Drain and set aside.
Stir flour into the sauteed onion; increase heat to medium and cook for 1 minute. Pour in milk and heavy cream in a steady stream, stirring or whisking to combine. Bring mixture to a simmer; cook until thickened, about 8 minutes.
Stir Cheddar cheese, Neufchatel cheese, hot pickled red peppers, and Worcestershire sauce into the milk mixture until fully melted and combined. Season with salt and pepper. Add pasta and toss carefully to coat. Pour into a large casserole dish.
Combine Parmesan-Romano cheese, bread crumbs, and olive oil in a small bowl. Sprinkle over mac and cheese.
Bake in the preheated oven until cheese is bubbling and the top is golden, 25 to 35 minutes.
