For meat sauce, crumble ground beef into large skillet over medium high heat, brown and discard fat.
Stir in mushrooms, onion and garlic, cook until onions are soft.
Press tomatoes and juices through sieve into meat mixture, discard seeds.
Stir in 3/4 teaspoons salt, oregano, basil and 1/8 teaspoons pepper. Bring to a boil, cover and simmer for 40 minutes, stirring occasionally. uncover and simmer 15-20 minutes more until sauce thickens; set aside.
Add lasagna noodles to lg. pot of salted boiling water 1 at a time, allowing noodles to soften to fit in pot.
Cook 10 minutes or just until dente.
Drain and rinse noodles with cold water. Hang individually over sides of pot to prevent sticking. Set aside.
For cheese sauce, 1/2 cup butter in sauce pan, stir in flour, remaining salt, pepper, and nutmeg; cook and stir until bubbly. Whisk in milk, cook and stir until sauce thickens and bubbles. Cook ans stir 1 minute more. Stir in 1 cup mozzarella and 1/4 cup parmesan. Stir until smooth, set aside.
Preheat oven 350°F Spread remaining butter on bottom and sides of 12x8-inch baking dish. Pat noodles dry.
Arrange 3 noodles in single layer, overlapping slightly in bottom of dish.
Top with 1/2 meat mixture, spread evenly, spread 1/2 cheese mixture over meat mixture evenly.
Repeat layers once. Sprinkle spinach over last layer or cheese. Pat down slightly. Arrange last 3 lasagna noodles over spinach.
Mix remaining cheeses together and sprinkle cheeses evenly on top of lasagna to cover completely.
Bake 40 minutes or until top is golden brown and sides are bubbly.