Preheat the oven to 350 degrees F. Grease a 9x13 inch pan.

Heat the olive oil in a large skillet over medium heat. When the oil shimmers, add the mushrooms and season with salt and pepper. Cook undisturbed for 5 minutes or until golden, stir and continue cooking until the mushrooms have caramelized, 3-5 minutes. Remove the mushrooms from the skillet to a plate.

Add the butter, garlic and oregano, salt, and pepper and cook 30 seconds or until fragrant. Whisk in the flour and cook for about 1 minute. Add the milk and broth. Bring to a boil and stir for 1 minute. Remove from heat and stir in the Raclette cheese and ½ cup of parmesan cheese. Stir until the cheese is fully melted and the sauce is smooth. Set the cheese sauce aside.

In a medium bowl combine the ricotta, spinach, and pesto.

Spread ¼ of the cheese sauce in the bottom of the prepared baking dish. Top with 3-4 lasagna sheets. Spread with ½ the ricotta cheese mixture, another ¼ of the cheese sauce, and half of the mushrooms. Place another 3-4 lasagna noodles on top and then top with the remaining ricotta cheese mixture, another ¼ of the cheese sauce, and the remaining mushrooms. Add the remaining lasagna noodles and pour the remaining cheese sauce over top. Top with a ½ cup of parmesan cheese.

Bake uncovered for 45 minutes or until the top has bubbled up and browned a bit. Let stand 10 minutes before serving.