In 4-quart saucepan, heat butter over medium heat. Add onions; cook 4 to 5 minutes, stirring occasionally, until tender. Add flour; cook and stir 1 minute. Add broth; heat to boiling, stirring constantly. Add cauliflower and red pepper; return to boiling. Reduce heat to medium-low; simmer uncovered 9 to 11 minutes or until cauliflower is tender. Stir in whipping cream; cook until hot. Remove from heat; gradually stir in 2 1/2 cups of the cheese until melted.