Heat olive oil and 2 tbsp butter in a pan over medium-high heat
Add mushrooms and cook until golden and moisture evaporates
Add garlic and cook 30 seconds
Stir in spinach until wilted
Season with salt, pepper, and nutmeg, then set aside
Melt 4 tbsp butter in a saucepan
Whisk in flour and cook 1 minute
Gradually add warm milk, whisking constantly
Add cream, salt, pepper, and nutmeg
Simmer until thick and silky
Mix ricotta, egg, Parmesan, salt, and pepper
Preheat oven to 180°C / 350°F
In a baking dish, spread white sauce, then layer lasagna sheets, mushroom-spinach mixture, ricotta mixture, and mozzarella
Repeat layers 3-4 times
Finish with white sauce, mozzarella, and Parmesan
Cover with foil and bake 30 minutes
Uncover and bake 15-20 minutes until golden and bubbling
Let rest 10 minutes before slicing
Garnish with parsley and serve
