Combine milk and flour, stirring well with a whisk; set aside.
Heat a medium saucepan over medium-high heat. Coat pan with cooking spray. Add leek and garlic; sauté 2 minutes or until tender. Add spinach, and sauté 1 minute. Stir in milk mixture; cook 1 1/2 minutes or until slightly thick, stirring occasionally. Stir in cheese, salt, and pepper, stirring until smooth; cook 1 minute.