In a medium saucepan over medium heat, melt 4 tablespoons of unsalted butter until foamy. Add 4–6 minced garlic cloves and sauté for about 1 minute until fragrant. Stir in 1/4 cup of all-purpose flour and continue cooking for an additional minute until toasted, creating a roux with a light golden color.

Gradually whisk in 2 cups of half and half and 1 cup of whole milk, stirring continuously to avoid lumps. Keep the mixture on the heat for 2–3 minutes, or until it thickens to a creamy consistency. Mix in 1 cup of grated Parmesan cheese, along with kosher salt, black pepper, and a pinch of ground nutmeg. Set this creamy béchamel sauce aside.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add sliced baby Bella mushrooms, sautéing for about 3–4 minutes until they are tender and golden brown. Toss in the baby spinach and minced garlic, stirring until the spinach wilts, which should take about another 2–3 minutes. Allow the mixture to cool slightly.

In a mixing bowl, combine 15 ounces of whole milk ricotta cheese, 1/2 cup of grated Parmesan, 1 large egg, and a sprinkle of salt and pepper. Gently fold in the cooled mushroom and spinach mixture.

Preheat your oven to 375°F (190°C). In a greased 9x13-inch baking dish, spread a thin layer of béchamel sauce on the bottom. Layer over 3–4 oven-ready lasagna noodles, followed by half of the ricotta mixture, and a layer of shredded mozzarella cheese. Repeat these layers, finishing with the béchamel sauce and a generous topping of mozzarella.

Cover the lasagna with foil and bake in the preheated oven for 50 minutes. Remove the foil and bake for an additional 10 minutes, or until the top is golden and bubbly. Let the white lasagna with spinach and mushrooms rest for 15 minutes before slicing.