Place the frozen spinach in the microwave and cook it according to package instructions. Place it in a colander and squeeze out the most liquid you can. Set aside.
Make the creamy part: in a medium-sized skillet or saucepan, melt the butter and saute the onions and garlic until fragrant, about 2 minutes.
Pour in the white wine and simmer until reduced by half.
Add the milk, heavy cream, cream cheese, nutmeg, red pepper flakes, salt, and pepper to taste. Simmer until everything is well incorporated and smooth.
Add the spinach and parmesan cheese and cook until hot.