Melt the butter in a large soup pot over medium high heat
Sauté the onions in the butter until tender. Add the flour and whisk until
incorporated (mixture will be lumpy). Gradually whisk in the chicken broth and
stir until free of lumps.
Add the broccoli and simmer for 20 minutes until thickened.
Transfer soup to a blender and puree until creamy and most of the chunks a
broken down. (Be careful as soup is hot. Hold a dishtowel on top of the lid to
prevent burns from steam.)
Add the sour cream and season with salt and pepper. Stir to combine.
Serve with a sandwich or slice of whole grain toast.