Cook the fresh spinach in a large skillet over medium heat with a splash of water, tossing, for 3–4 minutes, until wilted. Work in batches if needed. (If using thawed frozen spinach, skip this step.)
Drain the spinach in a colander and press out as much liquid as you can. Roughly chop it and wipe out the skillet.
Melt the butter in the skillet over medium heat. Add the garlic and cook for 30 seconds. Whisk in the flour and cook for 1 minute.
Slowly whisk in the milk. Simmer for 2–3 minutes, whisking, until the sauce thickens.
Stir in the spinach, salt, pepper and nutmeg. Simmer for 2–3 minutes, until hot and creamy. Stir in the Parmesan, if using.