In a large skillet over medium heat, melt 1 tablespoon of butter. Add spinach in batches, wilting each batch before adding the next. Once wilted, transfer spinach to a colander to drain excess liquid and set aside.

In the same skillet, add the remaining butter, chopped onion, and garlic. Cook over medium heat until onions are translucent and garlic is fragrant, about 3-4 minutes.

Sprinkle flour over the cooked onions and garlic, stirring constantly for about 2 minutes to cook off the raw flour taste.

Gradually add milk while continuously stirring to avoid lumps. Bring the mixture to a simmer and cook until thickened, about 5 minutes. Season with salt, pepper, and a pinch of nutmeg.

Add the wilted spinach back into the skillet with the sauce. Stir in grated Parmesan cheese until well combined and cheese is melted. Adjust seasoning if necessary and serve warm.