Preheat your oven to 350 degrees F. Grease a 9×13 inch pan to ensure your lasagna doesn't stick and releases easily after baking.
In a large skillet, heat the olive oil over medium heat. When the oil shimmers, toss in the wild mushrooms and season with kosher salt and pepper. Cook undisturbed for about five minutes, until they turn golden. Stir and continue cooking until they caramelize, which should take an additional three to five minutes. Once they’re done, remove the mushrooms from the skillet and set them aside on a plate.
Next, add the butter to the skillet along with the garlic and oregano. Season with more kosher salt and pepper, cooking for about thirty seconds or until it becomes fragrant. Whisk in the flour and let it cook for about one minute to eliminate the raw taste.
Pour in the whole milk and vegetable broth. Bring this mixture to a boil, stirring continuously for about one minute until it thickens slightly. Remove the skillet from the heat and stir in the Raclette cheese and half a cup of parmesan cheese. Stir until the cheese is fully melted and your sauce is smooth. Set this delicious cheese sauce aside.
In a medium bowl, combine the ricotta cheese, thawed spinach, and basil pesto. Mix thoroughly until well blended.
Spread one-fourth of the cheese sauce in the bottom of your prepared baking dish. Layer on three to four lasagna sheets, followed by half of the ricotta mixture, another one-fourth of the cheese sauce, and half of the sautéed mushrooms. Layer on another three to four lasagna noodles.
On top of the second layer of noodles, spread the remaining ricotta mixture, followed by another one-fourth of the cheese sauce and the remaining mushrooms. Finish by layering on the remaining lasagna noodles and pouring the rest of the cheese sauce on top. Sprinkle the remaining half cup of parmesan cheese on the very top.
Bake the lasagna uncovered for forty-five minutes, or until the top is bubbly and golden brown. Let it sit for about ten minutes before serving. This resting period allows the layers to set, making it easier to cut.
