Preheat the oven to 375°F (190°C).

Cook the lasagna noodles according to package directions. Drain, lay flat on a sheet of parchment or lightly oiled tray, and set aside.

Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 4–5 minutes. Add the minced garlic and cook for 30 seconds until fragrant.

Add the sliced mushrooms. Cook, stirring occasionally, until they release their moisture and begin to brown, about 6–8 minutes.

Stir in the chopped spinach. Cook until wilted and most liquid has evaporated. Season with salt, pepper, and dried oregano. Remove from heat and let cool slightly.

In a medium bowl, combine the ricotta cheese and egg. Mix until smooth and season with a pinch of salt and pepper.

Spread a thin layer of béchamel sauce on the bottom of a 9×13-inch baking dish. Lay three lasagna noodles across the sauce.

Spread one-third of the ricotta mixture over the noodles. Spoon half of the mushroom-spinach mixture on top of the ricotta, then sprinkle about 2/3 cup shredded mozzarella.

Repeat: add three noodles, another layer of ricotta, remaining mushroom-spinach, and another 2/3 cup mozzarella.

Top with the final three noodles. Pour remaining béchamel evenly over the top and finish with the remaining mozzarella and the grated Parmesan.

Cover the dish tightly with aluminum foil and bake for 25 minutes.

Remove the foil and bake another 15 minutes, until the top is golden and bubbly.

Let the lasagna rest for 8–10 minutes before slicing to allow the layers to set.