* Coat a large nonstick skillet with cooking spray and set pan over medium-high heat. Add onion, mushrooms and garlic; sauté until mushroom liquid cooks off, about 5 to 7 minutes. Add spinach; cook until liquid evaporates, about 2 minutes more. Remove skillet from heat and transfer mixture to a large bowl. Add ricotta, mozzarella, egg white and nutmeg; mix well to combine.
* Mix together tomato sauce and diced tomatoes in another bowl. Spoon 3/4 cup of tomato mixture into bottom of an 11 X 7-inch baking dish. Top with 3 uncooked lasagna noodles. Top with 3/4 cup more of tomato mixture and 1/2 of cheese mixture; spread cheese mixture to make an even layer. Top with 3 more noodles and press noodles slightly into cheese mixture. Top noodles with 3/4 cup more of tomato mixture and remaining cheese mixture. Top with remaining 3 noodles, pressing noodles slightly into cheese mixture. Top with remaining tomato mixture. Sprinkle with grated cheese topping.
* Bake until lasagna filling is bubbly and top is golden, about 40 minutes. Let stand 10 minutes before slicing into 6 pieces.