Bring a large pot of water to a boil, add the spinach, and cook for 4 to 5 minutes, until tender. Drain well and cool.
In a large sauté pan, sauté the pancetta, onion, and garlic together for 10 minutes, or until the onion is tender. Remove the pan from the heat and add the flour, salt, and pepper, mixing well. Return the pan to the heat and gradually add the milk, stirring until the mixture thickens. Add the spinach and mix well.