Heat 2 tablespoons of butter in a large, nonstick skillet over medium heat. Once melted and foamy, add the onions and cook for 2 minutes, stirring frequently.
Add the spinach, garlic, salt, pepper, and nutmeg. Stir well to combine and cook for 2 to 2½ minutes, stirring frequently.
Add the remaining 2 tablespoons of butter and let it melt.
Sprinkle the flour into the skillet, stirring constantly until fully incorporated. Cook for 1½ to 2 minutes, stirring frequently, to remove the raw flour taste.
Gradually add the milk, stirring vigorously with a wooden spoon until fully incorporated.
Bring the sauce to a simmer; this will take 1 to 2 minutes.
Reduce the heat to medium-low, cover, and cook for 5 minutes, stirring occasionally, until the sauce has thickened.
Lower the heat to low, then add the Parmesan cheese and stir until combined.
Taste and adjust the salt, if needed. As a reference, we added a pinch to ours.
Leave the creamed spinach uncovered; it will thicken further as it cools.