In a saucepan, saut� onions in butter until tender.
Stir in flour and salt until blended.
Gradually add broth, bring to a boil and cook, stirring constantly for 2 minutes or until thickened.
Reduce heat and add cauliflower; simmer for 2 minutes.
Add the milk and cheese; cook and stir until cheese is melted. Stir in sherry if desired. Garnish with chives.