1. For meat sauce, crumble ground beef into large skillet over medium-high heat. Brown 8 to 10 minutes, stirring to separate meat, until meat loses its pink color. Spoon off and discard fat.
2. Stir in mushrooms, onion and garlic; cook over medium heat 5 minutes or until onion is soft.
3. Press tomatoes and juice through sieve into meat mixture; discard seeds.
4. Stir in 3/4 teaspoon salt, oregano, basil and 1/8 teaspoon pepper. Bring to a boil over medium-high heat; reduce heat to low. Cover and simmer 40 minutes, stirring occasionally. Uncover and simmer 15 to 20 minutes more until sauce thickens. Set aside.
5. Add lasagna noodles to large pot of boiling salted water, 1 at a time, allowing noodles to soften and fit into pot. Cook 10 minutes or just until al dente.
6. Drain noodles; rinse with cold water. Drain again; hang individually over pot rim to prevent sticking. Set aside.
7. For cheese sauce, melt 1/4 cup butter in medium saucepan over medium heat. Stir in flour, remaining 1/2 teaspoon salt, remaining 1/8 teaspoon pepper and nutmeg; cook and stir until bubbly. Whisk in milk; cook and stir until sauce thickens and bubbles. Cook and stir 1 minute more. Remove from heat. Stir in 1 cup mozzarella and 1/4 cup Parmesan cheeses. Stir until smooth. Set aside.
8. Preheat oven to 350 degrees. Spread remaining 1 tablespoon butter on bottom and sides of 12 x 8 inch baking dish. If noodles are still wet, pat dry with a clean kitchen (not paper) towel.
9. Arrange lasagna noodles in a single layer, in the bottom of baking dish.
10. Top with 1/2 of the meat sauce; spread evenly. Spread 1/2 of the cheese sauce over meat sauce in an even layer.
11. Repeat layers once, lasagna noodles, remaining meat sauce and remaining cheese sauce. Sprinkle spinach over cheese sauce in an even layer; pat down lightly. Arrange another layer of lasagna noodles over spinach.
12. Mix remaining 1/2 cup mozzarella and remaining 1/4 cup Parmesan cheeses. Sprinkle cheeses evenly on top of lasagna to completely cover lasagna noodles.
13. Bake 40 minutes or until top is golden and edges are bubbly. Let lasagna stand 10 minutes before serving. Garnish as desired.