Make the béchamel sauce: Heat the butter in large saucepan until bubbly, but guard that it doesn't brown. Whisk in the flour, salt and pepper to make a paste. Whisk in hot milk all at once and cook over medium heat, stirring constantly, until thickened and smooth. Remove from heat; stir in nutmeg, Parmesan and spinach, set aside. Stir in eggs once sauce has cooled a bit.
For the sausage: In large skillet crumble and brown sausage over medium to medium-high heat. Saute garlic, peppers and mushrooms with the sausage until sausage is done and vegetables are tender, 7-8 minutes. Remover from heat and set aside.
To assemble: Heat oven 375*. Spray 9x13x2 pan with vegetable cooking spray. Layer in the following order:
1. Spread cups tomato sauce in the bottom of the dish. Cover with 4 pasta sheets, making sure they do not touch.
2. spread half of béchamel over the noodles, half of sausage-pepper mixture, 1 1/2 cups mozzarella, 1 1/2 cups tomato sauce.
3. Layer 4 more noodles, the remaining béchamel, sausage, 1 1/2 cups mozzarella and 1 1/2 cups tomato sauce.
4. Lay on 4 more noodles, the remaining tomato sauce and remaining mozzarella. Top with Parmesan.
Cover dish with heavy foil, tenting slightly to prevent foil from touching top layer. Bake in the middle of the oven for 20 minutes. Remove the foil and bake 20 minutes longer, until the top is bubbling and golden. Let stand 5-10 minutes before serving. Serves 10 generously.
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