Preheat the oven to 375 degrees F and position a rack in the top third of the oven.

For the sauce: In a medium saucepan, over medium heat, melt the butter. Stir in the flour and cook for 2 minutes. Whisk in the milk, 1/2 cup at a time. Season with salt, white pepper and nutmeg. Cook, stirring constantly for 4 to 6 minutes. Remove from the heat and whisk in 1/2 cup of the grated cheese. Cover and set aside; keep warm.

In a large sauté pan, over medium heat, add the oil. When the oil is hot, add the fennel and bell pepper and season lightly with salt and pepper. Sauté for 3 to 4 minutes, or until the vegetables are soft. Add the sausage and continue to sauté for 3 to 4 minutes, until lightly browned. Remove from the heat.

In a large mixing bowl, combine the pasta, sausage mixture and béchamel sauce. Mix well. Pour into a greased baking dish. Sprinkle the top with the remaining cheese and place in the oven on the top rack. Bake until the top is golden brown and bubbly, usually 20 to 25 minutes. Remove from the oven and cool slightly. Spoon onto serving plates and garnish with fresh basil.