Step 1: Prepare the Vegetable Filling

In a large skillet, heat olive oil over medium heat.

Sauté chopped onion until soft, about 3–4 minutes.

Stir in the garlic and cook for another minute until fragrant.

Add mushrooms, season with salt, pepper, and thyme (if using). Sauté until mushrooms are golden and have released their moisture, about 7–8 minutes.

Add spinach and cook until wilted. If using frozen spinach, squeeze out excess moisture before adding. Remove from heat and set aside.

Step 2: Make the Béchamel Sauce

In a saucepan, melt butter over medium heat.

Whisk in the flour and cook, stirring constantly, for 2 minutes to remove raw flour taste.

Gradually whisk in warm milk to prevent lumps. Continue stirring until the sauce thickens, about 5–7 minutes.

Season with salt, pepper, and nutmeg. Stir in the grated Parmesan until melted. Set aside.

Step 3: Assemble the Lasagna

Preheat oven to 375°F (190°C).

In a 9×13 inch baking dish, spread a thin layer of béchamel sauce on the bottom.

Layer 3–4 lasagna noodles over the sauce.

Add 1/3 of the spinach-mushroom mixture, pour over béchamel, and sprinkle with mozzarella.

Repeat layers, ending with béchamel on top. Sprinkle with remaining mozzarella and Parmesan cheese.

Step 4: Bake the Lasagna

Cover the dish with foil and bake for 25 minutes.

Remove foil and bake for another 15 minutes, or until the top is golden and bubbly.

Let the lasagna rest for 10–15 minutes before slicing. Garnish with chopped parsley if desired.