In a large skillet, melt 1 tablespoon of butter over medium heat. Add the chopped onion and cook until translucent, about 4 minutes.
Stir in the garlic and cook for an additional minute until fragrant.
Add the remaining tablespoon of butter, and once melted, stir in the flour. Cook for about 1-2 minutes, stirring frequently.
Gradually whisk in the milk and bring the mixture to a simmer. Cook until thickened, about 3-4 minutes.
Add the spinach to the skillet and cook until it has wilted down and is coated with the sauce, about 3 minutes.
Season with nutmeg, salt, and pepper to taste. Serve warm.