Use 2 (8-inch) square baking dishes so you have the option of baking both for a large gathering or baking one and freezing one for later.
Cook sausage and mushrooms in a lightly greased large Dutch oven over medium-high heat, stirring occasionally, 10 to 12 minutes or until meat crumbles and is no longer pink. Drain and return to Dutch oven. Stir in pasta sauce.
Drain spinach well, pressing between paper towels. Stir together spinach, ricotta cheese, egg, and Parmesan cheese in a large bowl until blended.
Arrange 2 lasagne noodles in a single layer in a lightly greased 8-inch square baking dish; layer evenly with 3/4 cup spinach mixture, 1 cup sausage mixture, and 1/2 cup cheese blend. Repeat layers twice, gently pressing layers to compact. Top with 2 noodles, 1 cup sausage mixture, and 1/2 cup cheese blend. (Dish will be full.) Repeat procedure with remaining ingredients in another lightly greased 8-inch square baking dish. Place both dishes on an aluminum foil-lined baking sheet.
Bake, covered, at 350° for 40 minutes; uncover and bake 8 to 10 more minutes or until bubbly. Let stand 10 minutes before serving.
Note: To freeze, prepare as directed through Step Cover baking dishes tightly with aluminum foil. Freeze up to 1 month. Thaw overnight in the refrigerator. Let stand at room temperature 30 minutes. Bake, covered, at 350° for 1 hour. Uncover and bake 15 more minutes or until bubbly.