In batches, saute the spinach in a large saute pan over high heat until wilted, 2 to 3 minutes. Transfer to a plate and set aside.
In the same pan, heat the olive oil over medium heat. Add the onions and garlic; cook until translucent, 5 to 8 minutes. Add the butter and flour; cook for 2 minutes. Slowly whisk in the milk and bring to a boil (it will thicken as comes to a boil). Turn off the heat and stir in the Parmesan, nutmeg and sauteed spinach.
Serve immediately or hold in a low oven.