Bring a large pot of salted water to a boil and add the lasagna noodles. Cook according to package instructions until al dente, about 8–10 minutes. Drain and lay flat on a towel.

In a skillet, heat olive oil over medium heat. Add diced onion and sauté until softened, about 3 minutes. Stir in minced garlic and cook for an additional minute. Add sliced mushrooms and sauté until golden brown, about 5–7 minutes. Mix in chopped spinach until wilted, season with Italian seasoning, salt, and pepper. Transfer to a bowl to cool.

In a mixing bowl, combine ricotta cheese, beaten egg, and grated Parmesan cheese. Stir until smooth, then fold in the cooled vegetable mixture.

Preheat oven to 375°F (190°C).

Spread 1/2 cup of marinara sauce in a baking dish. Take one lasagna noodle, spread a portion of filling along the length, roll it up tightly, seam-side down, place in the baking dish. Repeat with remaining noodles.

Pour remaining marinara sauce over the rolls and sprinkle shredded mozzarella cheese on top. Cover with foil and bake for 25 minutes, remove foil and bake for 10 more minutes until cheese is golden brown.

Let rest for 5 minutes before serving. Pair with side salad or garlic bread.