Set a large skillet on the stovetop on medium-high heat. Add the butter to the pan, then toss in the diced onion and minced garlic once it has melted.
Stir it in with the onion mixture, and as it wilts, toss in some more. Continue wilting the spinach in the skillet until all of it is cooked.
Mix the flour into the wilted spinach to thicken it, then sprinkle in the salt, pepper, and nutmeg. Stir everything to combine, then lower the burner heat to medium.
Stir the contents of the pan until the cheese has completely melted.
"Even though it's traditional to serve creamed spinach with steak or beef, this really goes well with anything!