Heat olive oil in a large pan
Sauté onion until soft, add garlic
Add mushrooms and cook until browned and moisture evaporates
Season with thyme, salt, and pepper
Add spinach and cook until wilted
Remove from heat
Melt butter in a saucepan
Whisk in flour and cook 1 minute
Slowly whisk in warm milk until smooth
Cook until thick
Season with nutmeg, salt, and white pepper
Preheat oven to 180°C (350°F)
Spread a thin layer of white sauce in a baking dish
Layer lasagna sheets, spinach-mushroom filling, ricotta (if using), mozzarella, and sauce
Repeat layers
Finish with sauce, mozzarella, and Parmesan
Cover with foil and bake 30 minutes
Uncover and bake 15–20 minutes until golden and bubbly
Rest 10 minutes before slicing
Serve with garlic bread or green salad
