In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat. Add 2 minced garlic cloves and sauté until fragrant, about 1 minute. Whisk in ¼ cup of all-purpose flour, stirring constantly until absorbed, which should take about 2 minutes. Gradually add 1 cup of half and half and 1 cup of whole milk, continuously whisking until the mixture thickens, approximately 5-7 minutes. Stir in 1 cup of grated Parmesan, salt, pepper, and a pinch of nutmeg, then set aside.

In a skillet, heat 2 tablespoons of olive oil over medium heat. Add 8 ounces of sliced baby bella mushrooms and sauté until they become lightly browned, around 5 minutes. Next, add another minced garlic clove and 2 cups of fresh baby spinach, cooking until the spinach wilts, which should take about 3 minutes. Once done, remove from heat and allow the mixture to cool slightly.

In a mixing bowl, combine 15 ounces of whole milk ricotta cheese, 2 large eggs, ½ cup of the remaining grated Parmesan, and a sprinkle of salt and pepper. Mix until smooth, and then fold in the cooled mushroom and spinach combination.

Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish. Begin layering by spreading a thin layer of the béchamel sauce on the bottom. Add the first layer of oven-ready lasagna noodles, followed by a layer of the ricotta mixture, a sprinkle of shredded mozzarella cheese, and more béchamel sauce. Repeat the layers, finishing with a final layer of noodles topped with béchamel and mozzarella.

Cover the assembled lasagna tightly with foil. Bake in the preheated oven for 50 minutes. After this time, remove the foil and bake uncovered for an additional 10 minutes. Let cool for about 15 minutes before slicing to ensure clean cuts.