Thaw spinach. Squeeze dry hands over sink hard. Dice onion knife. Mince garlic. Shred cheddar box grater.
Melt 4 tbsp butter large skillet medium heat wooden spoon. Add onion garlic. Stir 2 minutes soft fragrant. Whisk flour 2 minutes bubbly blond roux.
Whisk milk slow 3 minutes thickens coats back spoon. Stir cheddar handfuls melts smooth 2 minutes. Add Parmesan, salt, nutmeg wooden spoon blends.
Add spinach. Stir wooden spoon even coats. Simmer low 5 minutes thick bubbly cheese pulls. Serve hot stretchy.