Preheat the oven to 375°F and grease a large casserole dish.
In a food processor, pulse the leftover potato buns and paprika into fine breadcrumbs. Season with salt and pepper, and set aside.
Bring a large pot of salted water to a boil. Add the pasta and cook according to the package directions until al dente. Reserve 1/2 cup of the starchy cooking water and then drain the pasta. Set the pot aside.
In a large skillet, melt 2 tablespoons of the butter over medium-low heat. Add the onions and honey, and cook, stirring occasionally, until the onions are golden and caramelized, 15-20 minutes. Season with a pinch each of salt and pepper.
While the onions are caramelizing, make your roux: In the pasta pot over medium heat, add the remaining 2 tablespoons butter and the flour. Cook, stirring occasionally, for about 3 minutes. Slowly pour the warm milk into the roux, whisking constantly to avoid lumps. Cook, stirring frequently, until the sauce is thick—about 3-5 minutes.
Remove the sauce from heat and add in both cheeses and nutmeg, and stir until melted and smooth. Season to taste with salt and pepper. Add the noodles, caramelized red onions and chicken to the pot, and stir, using the starchy cooking water to thin if necessary, until all of the ingredients are thoroughly combined.
Pour the mixture into the casserole dish and sprinkle the parmesan and potato bun crumbs evenly over the top. Bake the casserole until the cheese is bubbly, about 30 minutes, then broil until the top is golden and crusty, about 2-3 minutes. Garnish with the chopped parsley.
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