Bring a large pot of water to a boil.
Blanch the spinach for 30 to 60 seconds.
Immediately plunge the spinach into a bowl filled with ice water.
Drain well, pat dry, and set aside.
Place the butter and mascarpone in a saucepan, and add the blanched spinach and combine.
Add salt, pepper to taste and nutmeg.
Gently heat until it begins to simmer.
Then mix in the Parmesan.
Serve topped with more grated Parmesan.
Optionally, you can place under the broiler for 2 to 3 minutes, until crisp and lightly golden.
Serve immediately or let it cool down and store in the fridge for up to 3 days.