Cook the leeks in oil over medium heat in a large sauce pan for about 5 minutes, or until they are soft
Add the cauliflower, water, 2 cups of milk, bay leaf, salt and pepper. Bring this to a boil while stirring it
When it boils, remove the bay leaf, reduce heat, cover and simmer until the cauliflower is soft, about 8 minutes
Whisk together the flour and ½ cup of milk, add this to the simmering mixture when the cauliflower is soft. Stir until it becomes thicker
Turn off the heat and add the lemon juice and cheese, stir until the cheese is melted
Ladle into bowls and serve!