Cheddar Cauliflower Soup

Heat oil in a large saucepan over medium heat. Add leeks and cook, stirring, until very soft, about 5 minutes. Add cauliflower, 2 cups milk, water, bay leaf, salt and pepper. Bring to a boil over medium-high heat, stirring often. Reduce heat to a simmer, cover and cook, stirring occasionally, until the cauliflower is soft, about 8 minutes. Meanwhile, whisk the remaining 1/2 cup milk and flour in a small bowl. When the cauliflower is soft, remove the bay leaf and stir in the milk mixture. Cook over medium-high heat, stirring, until the soup has thickened slightly, about 2 minutes more. Remove from the heat. Stir in cheese and lemon juice.