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

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

Add the butter, garlic, oregano, salt, and pepper to the skillet and cook for 30 seconds 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 1/2 cup of parmesan cheese until melted and smooth.

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

Spread 1/4 of the cheese sauce in the bottom of the prepared baking dish. Top with 3-4 lasagna sheets. Spread with 1/2 of the ricotta cheese mixture, another 1/4 of the cheese sauce, and half of the mushrooms.

Place another 3-4 lasagna noodles on top, then top with the remaining ricotta cheese mixture, another 1/4 of the cheese sauce, and the remaining mushrooms. Add the remaining lasagna noodles and pour the remaining cheese sauce over top. Top with the remaining 1/2 cup of parmesan cheese.

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