Preheat oven to 350 degrees F.
Prepare pasta based on package instructions. Drain, separate and set aside.
In a large pan, heat 4 Tbsp. olive oil over medium-high heat. Add mushrooms and sauté 3-4 minutes. Add spinach and remove the pan from heat.
Toss the spinach mixture until the spinach has wilted. Transfer to bowl and blend in ricotta, 12 oz. mozzarella, Parmesan, eggs and seasonings.
Spread 1 cup sauce each in two ungreased 9×13 inch baking dishes.
Stuff manicotti shells with cheese mixture (fill plastic bag and cut corner off to pipe in cheese mixture); arrange over sauce. Top both dishes with remaining sauce and mozzarella.
Cover and bake at 350 degrees F for 45-50 minutes.