In a medium saucepan over medium heat, melt the butter. Once frothy, stir in the minced garlic and sauté for about 1 minute until fragrant. Gradually whisk in flour, cooking until a roux forms.

Add the half and half and whole milk slowly, whisking continuously until thickened; about 5–7 minutes. Stir in grated Parmesan, salt, pepper, and nutmeg.

In a large skillet, heat olive oil over medium heat. Add sliced mushrooms and sauté for 5-7 minutes until tender. Stir in minced garlic, cooking for another minute before adding spinach. Cook until wilted.

In a large bowl, combine ricotta cheese, mozzarella cheese, and grated Parmesan cheese. Add in the egg and mix thoroughly. Season with salt and pepper, then fold in sautéed mushrooms and spinach.

Preheat your oven to 375°F (190°C). Grease a baking dish with olive oil. Spread a thin layer of béchamel sauce. Layer lasagna noodles, half of the cheese filling, and béchamel sauce. Repeat and finish with noodles topped with remaining béchamel and cheese.

Cover with foil and bake for 50 minutes. Remove the foil and bake an additional 10–15 minutes until the top is golden brown and bubbly.

Let the lasagna rest for about 15 minutes before slicing to allow the layers to set.