Fully thaw the frozen spinach, then squeeze out the excess moisture. Set the spinach aside.
Melt the butter in a large skillet over medium heat. Add the minced garlic and continue to sauté over medium heat for about one minute.
Add the flour to the skillet. Whisk the flour and butter together and continue to cook and stir for about two minutes more.
Whisk the milk into the flour and butter. Continue whisking until the sauce comes up to a simmer, at which point it will thicken.
Add the Parmesan to the sauce and continue to whisk and cook until the Parmesan melts into the sauce (1-2 minutes). Season the sauce with nutmeg, salt, and pepper.
Finally, stir the chopped spinach into the white sauce until evenly combined. Give the spinach one final taste and adjust the salt as needed. Enjoy!
* To use fresh spinach, roughly chop 12 oz. fresh baby spinach. Sauté the spinach in a skillet with a little oil until most of the moisture has cooked out.