Melt butter in a large, heavy-bottomed skillet over medium-low heat. Add onion and cook for 2-3 minutes, stirring until onion is slightly softened. Add flour and continue to stir, coating onion thoroughly. Add garlic and cook for 1 minute more.

Slowly whisk in milk, breaking up flour/onion/garlic mixture. Increase heat to medium and bring to a boil, stirring constantly until you have a smooth white sauce. Reduce heat to low and add nutmeg and cheese. Remove from heat.

Heat olive oil in a large stockpot. Add spinach and stir constantly to saute until spinach is cooked down and wilted (the volume will dramatically decrease). Remove from heat and add to cheese mixture. Stir thoroughly to incorporate. Serve immediately.