Preheat the oven to 375°F and bring a large pot of salted water to a boil. Cook the lasagna noodles according to package directions until al dente, then drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 minutes until softened.

Stir in the minced garlic and sliced mushrooms. Cook for 5 minutes until the mushrooms release their liquid and begin to brown.

Add the fresh spinach in batches, stirring until it wilts completely. Season with salt, pepper, and dried oregano, then remove the skillet from the heat.

In a bowl, mix the ricotta cheese with half of the grated Parmesan and a pinch of black pepper.

Spread 1 cup of marinara sauce across the bottom of a 9x13 baking dish. Layer 3 noodles on top, followed by half the ricotta mixture, half the spinach-mushroom filling, and ½ cup mozzarella.

Repeat the layers once more with another 3 noodles, the remaining ricotta, the rest of the filling, and another ½ cup mozzarella.

Top with the final 3 noodles, the remaining marinara sauce, the rest of the mozzarella, and the remaining Parmesan.

Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for 10 more minutes until the cheese is bubbly and lightly browned. Let it rest for 5 minutes before slicing.