Wash leeks very carefully to remove all grit. I usually cut them horizontally and separate the rings to make sure no dirt remains.
Coarsely chop them when washed.
In a medium soup pot, melt butter and add flour on low flame.
Using a wooden spoon, stir and cook 1 to 2 minutes.
Add broth, leeks, and cauliflower and bring to a boil.
Cover and simmer on low for about 20 minutes, until cauliflower is soft.
Using an immersion blender, blend the soup until smooth adjusting the salt and pepper to taste.
Serve immediately.
Notes
Makes about 8 cups.Instant Pot VariationSauté the leeks in butter on Sauté mode. Add flour and stir. Add cauliflower, broth, salt, and pepper. Cook on High Pressure for 5 minutes, quick release, then blend.