Preheat oven to 350°F.
Bring a large pot of lightly salted water to a boil. Add lasagna noodles and cook for 8-10 minutes or until al dente. Drain the noodles and set aside.
15 lasagna noodles
Blanch the spinach in a shallow pan of water for 2 minutes. Drain, then press out any excess liquid. Allow to cool, then chop and set aside.
9 ounces fresh spinach
Heat the olive oil in a skillet set over medium-high heat. Add the mushrooms, onions, garlic, ½ teaspoon salt, and ½ teaspoon pepper. Cook for 5 minutes, until onions are tender. Drain excess liquid, transfer to a bowl, and allow to cool.
2 tablespoons olive oil, 8 ounces cremini mushrooms, 1 cup onion, 3 cloves garlic, 1 ½ teaspoons salt, 1 ½ teaspoons ground black pepper
In a large bowl, combine the ricotta cheese, ½ cup Parmesan cheese, cooled spinach, Italian seasoning, egg, and remaining salt and pepper. Mix well.
3 cups ricotta cheese, 1 ½ cups Parmesan cheese, 2 teaspoons Italian seasoning, 1 large egg
Spread ½ cup marinara sauce on the bottom of a 9x13-inch casserole dish. Layer 4 lasagna noodles on top of the sauce so they’re slightly overlapping. Top with ⅓ of the cheese/spinach mixture, ⅓ of the mushroom mixture, ⅓ of the remaining sauce, 1 cup of the mozzarella cheese and ⅓ cup of the Parmesan.
24 ounces marinara sauce, 3 cups mozzarella cheese
Repeat the layers two more times.
Cover with foil and bake for 45 minutes, then remove the foil and continue baking for 15 more minutes.
Let the baked lasagna cool for 10 minutes, garnish with parsley, slice, and serve.