Preheat oven to 350 degree F. Lightly grease an 8x8-inch baking dish. Place sheets of no boil pasta in hot water in a 9x13-inch baking dish; make sure pasta does not stick together while it soaks 10-15 minutes. Drain off water. Keep pasta separated so the sheets do not stick.
In a medium size bowl combine the ricotta cheese, cream, egg yolk, pinch of nutmeg, salt, spinach and 1/2 cup shredded mozzarella cheese.
In a medium size sauté pan heat the olive oil. Add the mushrooms and salt and pepper to taste. Sauté mushrooms until tender and moisture has evaporated.
Spread a small amount of sauce on the bottom of the baking dish. Place two sheets of lasagna side-by-side. Spread 1/3 ricotta mixture over pasta, spread 1/3 cooked mushrooms over ricotta and 1/3 spinach, and 1/2 cup of mozzarella.Top with 1/4 jar Butternut Squash Sauce. Repeat layers of ricotta, mushrooms, spinach, mozzarella and sauce two more times. Top with the last two sheets of lasagna pasta, remaining sauce and the remaining mozzarella cheese.
Bake in 350 degree F. oven for 30 minutes or until the cheese has melted and the sauce is bubbling. Allow dish to sit 10 minutes before serving.