Cook Lasagna Noodles: Bring a large pot of salted water to a boil. Add the lasagna noodles and cook according to package instructions until al dente. Drain and set aside.

Sauté Vegetables: In a large skillet, heat olive oil over medium heat. Add chopped onion and cook until translucent, about 3-4 minutes. Stir in minced garlic and sliced mushrooms, cooking until the mushrooms are tender, about 5 minutes.

Add Spinach and Chicken: Add fresh spinach to the skillet and cook until wilted. Stir in the shredded chicken, dried oregano, dried basil, salt, and pepper. Mix well and remove from heat.

Combine Ricotta: In a medium bowl, combine ricotta cheese, egg, salt, and pepper. Mix until well blended.

Layer Ingredients: Preheat the oven to 375°F (190°C). Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish. Place three lasagna noodles over the sauce. Spread half of the ricotta mixture over the noodles, followed by half of the chicken and vegetable mixture, and a third of the mozzarella cheese. Repeat the layers, finishing with the remaining noodles, marinara sauce, mozzarella cheese, and grated Parmesan cheese on top.

Bake Lasagna: Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden.

Cool and Slice: Allow the lasagna to cool for about 10 minutes before slicing. Serve warm.