Coat the bottom of a large skillet with the olive oil and place over MEDIUM heat. If using link sausage, remove it from the casings by slicing through one side with a sharp knife. Discard casings and place the sausage in heated skillet. Cook and stir, breaking up the sausage, until no longer pink. Drain as much grease from the pan as possible and return the pan to the heat. Add the garlic, ½ teaspoon salt and the pepper. Cook and stir for another minute or two and then remove the pan from the heat and set aside.
In a small mixing bowl, combine the ricotta, egg, remaining ½ teaspoon salt, and chopped basil. Set aside.
Remove the stems, chop the mushrooms and set aside.
Lightly coat the bottom of a deep, freezer safe 13- x 9-inch pan with 1 cup of marinara sauce. Place 4 lasagna sheets over the sauce, overlapping just slightly, if needed. Spoon ⅓ of the ricotta mixture over the lasagna sheets and spread it out evenly with a knife. Sprinkle ⅓ of the cooked sausage over the ricotta. Top with ⅓ of the chopped mushrooms, purposely allowing some to fill in edges of the pan around the lasagna. Sprinkle with ¼ of both the mozzarella and Parmesan and then pour 1 cup (no more) of marinara evenly over the top.
Layer as directed above 2 more times for a total of 3 layers.
Top with the remaining 4 lasagna sheets and pour the remaining marinara sauce over the top. Use a knife or spatula to smooth out the sauce over the pasta sheets. Sprinkle with remaining mozzarella and Parmesan. See Notes section below for instructions to bake immediately, or follow as directed to freeze the lasagna.