Bring 6 cups of water to a boil in a large sauce pan. Add frozen broccoli and onion and boil 10-12 minutes. Add cheese, salt, white pepper and garlic powder. Cook over medium heat, stirring constantly, until cheese is completely melted. Add half & half and butter and bring to a boil, still stirring constantly. Whisk flour into cold water until smooth, then slowly whisk into hot soup. Continue whisking until soup thickens to consistency of heavy cream.