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.