Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat. Add 2 tablespoons of minced garlic and sauté for about 1 minute until fragrant. Whisk in 1/4 cup of all-purpose flour to create a roux, stirring continuously for 2-3 minutes until golden. Gradually pour in 1 cup of half and half and 1 cup of whole milk, whisking constantly. Allow to thicken for about 5-7 minutes. Stir in 1/2 cup of grated Parmesan cheese, a pinch of kosher salt, black pepper, and ground nutmeg; set aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add 8 ounces of sliced baby Bella mushrooms and cook for 3-4 minutes until tender and browned. Add the reserved minced garlic and fresh baby spinach. Sauté until the spinach wilts, about 2-3 minutes. Transfer to a bowl and let it cool.

In a large mixing bowl, combine 15 ounces of ricotta cheese, 1 cup of mozzarella cheese, remaining 1/2 cup of Parmesan, and 2 beaten eggs. Mix thoroughly, then gently fold in the cooled mushroom and spinach mixture.

Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish. Spread 1 cup of the béchamel sauce on the bottom, layer with 3-4 lasagna noodles, half of the ricotta cheese mixture, and a sprinkling of 1/2 cup of mozzarella. Repeat layers, finishing with béchamel and top with mozzarella.

Cover with foil and bake for 50 minutes. Remove foil and bake uncovered for an additional 10 minutes until golden and bubbly. Let it rest for about 15 minutes before slicing.