Heat 2 tablespoons of butter in a nonstick saucepan or large skillet over medium heat. When the butter is melted and foamy add the onions. Cook for 2 minutes, stirring frequently.
Add the spinach, garlic, salt, pepper, and nutmeg, stir well to combine. Cook the spinach mixture for 2 - 2½ minutes, stirring frequently.
Add the remaining 2 tablespoons of butter and heat until melted.
Add the flour to the skillet and stir constantly until combined. Cook the flour for 1½ - 2 minutes to cook out the raw flour taste. Stir frequently.
Add the milk while stirring vigorously with a wooden spoon to incorporate.
Bring the sauce to a simmer, it will take 1-2 minutes.
Lower the heat to medium-low, cover and cook for 5 minutes until thickened. Stir occasionally.
Lower the heat to low and add the parmesan cheese, stir.
Taste and add salt if needed. As a reference we added a pinch to ours. Leave the creamed spinach uncovered, it will thicken further as it cools.