How to Make It
Preheat the oven to 400°. Lightly butter a 9-by-13-inch baking dish. On a baking sheet, toss the sweet potatoes with 1/4 cup of the olive oil, the thyme and 1 teaspoon of salt; season with pepper. Roast the potatoes for 20 minutes, stirring occasionally, until tender.
Meanwhile, cook the pasta in a pot of salted boiling water until almost al dente, 5 minutes for torchio and 10 minutes for rigatoni. Drain, rinse under cold water and return to the pot.
In a small saucepan, bring the milk just to a simmer. In a medium saucepan, melt the 2 tablespoons of butter. Add the flour and cook over moderately low heat, whisking frequently, until lightly golden, about 3 minutes. Whisk in the hot milk in 3 additions until incorporated. Cook over moderate heat, whisking often, until thickened, 10 to 15 minutes. Stir in the Fontina, the shredded Cantal, 1/4 cup of the Parmigiano and the nutmeg until the cheeses are melted. Stir in the heavy cream and season with salt and pepper. Stir the cheese sauce into the pasta.
In a medium bowl, toss the radicchio with the rosemary, the remaining olive oil and 1/2 teaspoon of salt. Season with pepper and toss, then add to the pasta and mix well. Spread half of the pasta in the prepared baking dish. Tuck in half each of the sweet potatoes, cubed Cantal and pecans. Repeat with the remaining pasta, sweet potatoes, Cantal and pecans. Sprinkle the remaining 1/2 cup of Parmigiano on top.
Cover the baking dish with foil and bake for 10 minutes. Uncover and bake for about 10 minutes longer, until the cheese is melted and the pasta is tender. Let the baked pasta stand for 15 minutes. Garnish with the slivered parsley and serve.
The unbaked pasta can be refrigerated overnight. Bring to room temperature before baking.