In large stockpot, melt butter over medium heat. Add onion cook until tender. Stir in broccoli, water & boullion cubes. Simmer until broccoli is tender. 10 -15 minutes.
Reduce heat, stir in cheese cubes until melted. Add milk and garlic powder. In small bowl, stir corn starch into 1/2 cup of water until dissolved. Stir into soup, cook until thick. Top with shredded cheese before serving.