Thaw and steam the spinach in the microwave according to package directions. Transfer the spinach to a colander, lined with a tea towel, and allow it to cool. When the spinach is cool enough to handle, squeeze out all the excess liquid; set aside.
In a large saute pan, melt the butter over medium heat. Add the chopped onions and sauté for 5-7 minutes until soft and translucent ( try not to brown). Add the garlic and cook for a few seconds until fragrant.
Add the nutmeg, mustard, black and cayenne pepper. Add the cream cheese, half n half and stir to combine. Simmer and stir until the cream cheese starts to melt. If the cream cheese doesn’t melt, use a whisk to help breakup any lumps.
When the cheese sauce begins to thicken add the spinach and parmesan cheese. Combine the cream sauce, parm and spinach, mix well. Bring it back to a simmer and heat through. Add a little milk or half and half if it needs more liquid. Taste and adjust the salt, pepper.
Serve immediately or transfer it to a covered dish, cool before refrigerating for later use. Eat within 2-3 days.