Cook spinach in microwave according to package directions. Remove excess water (squeeze carefully using cheese cloth, a tea towel, a fine mesh sieve, or a colander) and set aside.
In a large skillet over medium high heat, melt the butter. Add onion and sauté until translucent. Add garlic and cook for 30 seconds.
Add cream cheese and cook, stirring, until melted. Add heavy cream and cook for 2 minutes. Add mozzarella cheese, parmesan cheese, optional nutmeg, salt and pepper. Stir until combined.
Fold the drained spinach into the cream sauce and cook for an additional minute to reheat the spinach. Serve immediately!
