Bring a large pot of salted water to a boil. Add the spinach and cook for about 1 minute until it begins to wilt. Place into a bowl of cold water to stop the cooking immediately. Drain.
Heat up butter in a 12-inch skillet over medium heat until it melts.
Add the onion and garlic to the skillet and stir occasionally while cooking for 2 to 3 minutes until the onion is soft and translucent.
Add the flour to the skillet and stir constantly while cooking for about a minute until it becomes foamy.
Gradually whisk in the half and half and milk. Add the nutmeg, red chili flakes, salt, and pepper into the skillet. Bring the mixture to a boil, then lower the heat and let it simmer for 5 minutes until it slightly thickens.
Add the spinach and Parmesan cheese into the skillet and mix them in. Cook the mixture for another minute to heat everything through.
Serve and enjoy!
