Preheat the Oven: Set to 375°F (190°C) and lightly grease a baking dish.
Sauté the Vegetables: Heat olive oil in a skillet over medium heat. Add chopped onions and garlic, cooking until softened. Stir in mushrooms and spinach, cooking until tender. Season with salt and pepper.
Prepare the Sauce: In a bowl, whisk together cream of mushroom soup, milk, dried basil, and oregano.
Assemble the Lasagna: Spread a thin layer of sauce at the bottom of the dish. Add a layer of lasagna noodles, followed by ricotta cheese, sautéed vegetables, mozzarella, and Parmesan. Repeat layers until all ingredients are used, finishing with a layer of cheese on top.
Bake Until Golden: Cover with foil and bake for 25 minutes. Remove the foil and bake for another 15 minutes until the cheese is golden and bubbling.
Rest & Serve: Let the lasagna cool for 10 minutes before slicing. Garnish with fresh parsley and serve warm.