Brown sausage until cooked through and drain off excess fat. Stir in tomatoes and garlic and bring to a simmer. Cook covered on low for at least 30 minutes, longer if time allows. Stir in basil just before assembly.
Make the bechamel by melting butter in a medium sized sauce pan. Stir the flour into the butter, adding more flour if needed to create a toothpaste-like consistency. Cook for 1 minute, stirring constantly.
Using a whisk, stir in milk and continue to stir until it comes to a boil and is thickened. Remove from heat and stir in salt and nutmeg.
In a large casserole dish (9x13" or bigger) place half of the pasta followed by half of the tomato sauce and bechamel. Top with 1 C of mozzarella cheese.
Repeat the layering one more time with the remaining ingredients.
Bake at 350° for about 45 minutes or until the sauce is bubbly and the cheese is browned.