Cook onion and carrots in olive oil over medium heat for about 5 minutes, stirring occasionally, until onion is tender. Stir in mustard seed, salt and pepper.
Stir in broccoli, broth and water. Heat to boiling; reduce heat. Cover and simmer about 10 minutes or until broccoli is tender.
Remove the pan from the heat and blend using an immersion blender. Blend until smooth.
Return the pan to the burner. Heat over low heat just until hot. Stir in sour cream. Top with shredded cheese if desired.