In saucepan, melt butter over medium heat; cook onion for 5 to 7 minutes or until soft and lightly golden. Sprinkle in flour; cook, stirring, for 2 minutes or until smooth. Slowly whisk in milk until smooth; bring to boil, stirring constantly. Cook, stirring frequently, for 10 minutes or until sauce thickens slightly. Season with salt, pepper and nutmeg.
Preheat oven to 375° (190°C). Grease glass baking dish; set aside.
Place sweet potato in saucepan; pour in enough water to cover and bring to boil. Cook for about 15 minutes or until sweet potato is fork-tender; drain. Meanwhile, cook fusilli according to package directions; drain. In large skillet, heat 1/2 of the olive oil; cook sausages for 3 minutes per side or until browned. Transfer sausages to cutting board; slice into 1/4-inch (5 mm) slices diagonally. Drain fat from skillet; return sausages to skillet along with pasta and sweet potato. Add Béchamel Sauce, half of the Parmesan, and Cheddar; stir to combine. Transfer to prepared baking dish.
In separate bowl, combine bread crumbs, parsley, garlic and remaining Parmesan and olive oil; sprinkle over pasta. Cover and bake for 15 minutes; remove cover and bake for 3 to 5 minutes or until top is golden brown and crisp.