In a small saucepan over medium heat, warm the milk. In a medium saucepan over medium heat, melt the butter. Whisk the flour into the butter and cook for 2 minutes to make a roux. Whisk in the warm milk a bit at a time to avoid lumps. Cook until the sauce begins to thicken, 2 to 3 minutes. Add the pumpkin pie spice and a pinch of salt and pepper. Turn the heat to low, stir in the cheese, and let melt. Stir in the spinach and cook for another 2 to 3 minutes to heat it through. (Save 1 cup for Round 2 Recipe Spinach and Cheese Souffle).