Step 1: Prepare the Spinach and Mushroom Filling

1. Heat olive oil and butter in a large skillet over medium heat.
2. Add the mushrooms and sauté until they release their moisture and become golden brown, about 5–7 minutes.
3. Add the minced garlic, spinach, thyme, salt, and pepper. Cook until the spinach is wilted, about 2–3 minutes. Remove from heat and set aside.

Step 2: Make the White Sauce

1. In a medium saucepan, melt butter over medium heat.
2. Whisk in the flour and cook for 1–2 minutes, stirring constantly, until the mixture forms a smooth paste (roux).
3. Gradually pour in the milk, whisking continuously to avoid lumps.
4. Cook until the sauce thickens, about 5 minutes.
5. Stir in salt, pepper, and nutmeg (if using). Remove from heat and set aside.

Step 3: Assemble the Cheese Mixture

1. In a bowl, combine ricotta, mozzarella, and Parmesan cheese. Mix until well blended.

Step 4: Layer the Lasagna

1. Preheat your oven to 375°F (190°C).
2. Spread a thin layer of white sauce at the bottom of a 9×13-inch baking dish.
3. Lay down 3 lasagna sheets (or enough to cover the base).
4. Spread ⅓ of the spinach and mushroom mixture over the sheets.
5. Dollop and spread ⅓ of the cheese mixture over the vegetables.
6. Pour a ladle of white sauce over the cheese layer.
7. Repeat this layering process (sheets, vegetables, cheese, sauce) two more times.
8. Finish with a layer of lasagna sheets topped with white sauce. Sprinkle the shredded mozzarella and Parmesan for the topping.

Step 5: Bake the Lasagna

1. Cover the baking dish with foil (to prevent drying).
2. Bake in the preheated oven for 25 minutes.
3. Remove the foil and bake for an additional 15–20 minutes, or until the top is bubbly and golden.
4. Let the lasagna rest for 10–15 minutes before slicing and serving.