In bowl, combine porcini and 3/4 cup hot water, let stand 30 minutes. With slotted spoon, remove porcini, rinse to remove any grit. Chop and set aside. Strain soaking liquid through sieve lined with paper towel, set aside. Meanwhile, prepare lasagna noodles as label directs, but do not add salt to water, drain and rinse with cold running water. Return noodles to saucepot with cold water to cover. Set aside.
Prepare bÉchamel sauce: In 3-quart saucepan, heat milk to boiling over medium-high heat. Meanwhile, in 4-quart saucepan, melt 3 tablespoons margarine over medium heat. Stir in flour, cook, stirring, 1 minute. Gradually whisk in milk, nutmeg, 1/2 teaspoon salt, and 1/8 teaspoon pepper. Heat to boiling. Reduce heat to low, simmer 5 minutes, stirring.
In 12-inch skillet, melt remaining 2 tablespoons margarine over medium-high heat. Add shallots, cook 1 minute. Stir in white mushrooms, 1/2 teaspoon salt, and 1/8 teaspoon pepper and cook 10 minutes, or until liquid has evaporated. Stir in porcini and soaking liquid, cook until liquid has evaporated. Remove from heat, stir in parsley.
Preheat oven to 375 degrees F. In large bowl, mix ricotta, spinach, 1/4 cup Parmesan, 1/2 teaspoon salt, 1/4 teaspoon pepper, and 1/2 cup bÉchamel. Drain noodles on clean kitchen towels.
In 13" by 9" glass baking dish, spread 1/2 cup bÉchamel. Arrange 4 lasagna noodles over sauce, overlapping slightly. Top with half of mushroom mixture, 1 cup bÉchamel, 1/4 cup Parmesan, and 4 more noodles. Add all of ricotta mixture, 4 more noodles, remaining mushrooms, 1 more cup bÉchamel, and 1/4 cup Parmesan. Top with remaining noodles, bÉchamel, and Parmesan. Cover with foil, bake 30 minutes.
Uncover and bake 20 minutes longer, or until sauce is bubbly and top is lightly browned. Let stand 15 minutes for easier serving.
