Cook lasagna in a large quantity of boiling water until just barely tender; drain off most of water; add ice cubes to stop cooking. (Allow lasagna to stand in cold water to prevent pasta sticking together.) In a saucepan, brown meat with onion and garlic; drain off fat. Cook over low heat, covered, for about 5 min. Stir in mushrooms with liquid, tomato sauce, tomato paste, salt and oregano. Simmer 15 mins. Combine egg with spinach, cottage and parmesan cheeses. Spread one third of meat sauce in 13x9 in. baking dish. Cover quarter of lasagna (about 5 overlapping strips.) Alternate another one third of sauce and quarter of lasagna. Spread spinach mixture over pasta and cover with quarter lasagna strips. Top with remaining sauce and pasta. Arange mozzarella cheese on top. Bake at 180ºC for 30 mins. Let stand 10 mins. before serving.