Melt butter in medium saucepan and chopped onions until tender and translucent (about 3 minutes)

Add flour to onion and mix (about a minute)

Add chicken broth and cauliflower florets. Let it boil.

Once boiled, lower heat and let it simmer (about 15-20 mins)

Once the cauliflower is tender, add the watercress. Let it cook for a minute, until watercress is wilted.

Either transfer soup to blender or use immersion blender. I had to transfer it to a blender in two separate batches.

Add salt and pepper to taste and blend until smooth.