After the spinach has thawed, drain it well, pressing all of the liquid out through a sieve into a bowl. Do not discard the liquid as you will use some of this liquid later.
Drain the artichoke hearts and place them in a food processor. Add the spinach, mozzarella, cream cheese, salt, nutmeg and mayonnaise. Process until almost smooth. Add some of the spinach liquid, 1 Tablespoon at a time, to help achieve that smoothness. One to two Tablespoons is usually enough.
When ready to serve, place the dip in a microwave-safe bowl. Put the dip in the microwave and heat in 30 second intervals, stirring well in between each 30 second interval. The dip should be hot enough to let the cheese melt and add to the creaminess of the dip, but not too hot or you will overcook the spinach. Serve immediately.
