How to Make It
Preheat the oven to 375°F. In a large pot of boiling salted water, cook the pasta until al dente, then drain.
While the pasta is cooking, in a medium saucepan, heat the oil over medium high heat until hot, then whisk in the flour and cook, whisking, until the flour is fragrant, about 4 minutes. Whisk in the milk and 1/2 teaspoon each salt and pepper, then bring to a boil, whisking. Boil until the milk is slightly thickened, about 8 minutes. Remove from the heat and whisk in 11/2 cups of the cheddar and the parmesan cheeses until melted, then whisk in the chicken. Season the sauce with salt and pepper to taste.
Toss the pasta together with the sauce and transfer to a 3-quart baking dish and sprinkle with the remaining 1/2 cup cheddar cheese. Bake the casserole until the filling is bubbling, 25 to 30 minutes. Serve.