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, 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. Set aside and keep warm.
In a large saute pan, over medium heat, add the oil. When the oil is hot, add the fennel. Season with salt and pepper. Saute for 3 to 4 minutes or until soft. Add the sausage and continue to saute for 3 to 4 minutes. Remove from the heat.
In a large mixing bowl, combine the pasta, sausage mixture and Mornay sauce. Mix well. Pour into a greased 2-quart oval baking dish. Sprinkle the top with cheese and place in the oven on the top rack. Bake until the top is golden brown and bubbly, about 10 to 12 minutes. Remove from the oven and cool slightly. Spoon onto serving plates and garnish with fresh basil.