Bring a large pot of water to a boil and blanch the spinach for 30 seconds until wilted. Drain immediately and rinse under cold water. Squeeze out excess liquid and roughly chop.
In a large skillet over medium heat, melt the butter. Add the diced onion and cook until translucent, about 4–5 minutes.
Stir in the garlic and cook for another 30 seconds.
Sprinkle the flour over the onion mixture and cook, stirring constantly, for 1–2 minutes.
Slowly whisk in the milk and heavy cream until smooth. Bring to a simmer and cook until thickened, about 5 minutes.
Season with salt, pepper, and nutmeg.
Fold in the chopped spinach and cook for 2–3 minutes to heat through.
Stir in the Parmesan cheese until melted. Serve warm.