In a large pot, combine chicken broth, onion, cauliflower and broccoli. Bring to a boil and reduce heat to low. Continue to simmer until vegetables are tender. Add chicken bouillon, salt and pepper.
In a bowl, combine flour and milk. Whisk until there are no lumps. Add mixture to soup, stirring continuously as soup thickens. Blend with an immersion blender, or in batches in a food processor, until smooth. Stir in Cheddar cheese until completely melted.
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