Fry the onion. Add butter to a dry frying pan. Melt over low heat. Stir the butter constantly. Add the leek to the butter. Fry for 5-7 minutes until translucent. Remove the frying pan from the heat.
Cook the broccoli. Pour water into a saucepan and set it on medium heat. Add the broccoli. Season with salt and pepper. Cover with a lid. Cook the broccoli until soft. Drain the water from the saucepan.
Finish cooking. Add the fried onions to the pan with the broccoli. Pour in the cream. Blend the contents with an immersion blender until smooth.