Preheat oven to 350º. Cook pasta according to package directions, and drain. Return to pot, and add sweet potatoes
In a large saucepan, melt butter over medium heat. Whisk in flour, salt, and pepper, and cook for 2 minutes. Whisk in cream and milk; cook, stirring frequently, until thickened, about 3 minutes. Stir in fontina 1/4 cup at a time. Stir cheese sauce into pasta.
Transfer macaroni and cheese to a 2-quart baking dish. In a small bowl, combine bread crumbs, Parmesan, pecans, and oil; sprinkle over pasta.
Bake for 30 minutes or until bubbly and golden. Let stand 10 minutes before serving.