In a large pan, bring 6 cups of water to a boil. Add broccoli and onions; boil for 10 to 12 minutes. Add cheese, salt, pepper, and garlic powder. Cook over medium heat, stirring constantly, until cheese melts. Add milk, cream, and butter. Heat to boiling, stirring constantly.

In a bowl, combine cold water and flour and mix until smooth. Add slowly to the hot soup, stirring rapidly. Continue to cook, stirring constantly, until the soup is the consistency of heavy cream. Serve hot.