Gather all ingredients.
Melt butter in a stockpot over medium heat. Add onion and cook, stirring occasionally, until softened. Stir in broccoli.
Add broth and simmer until broccoli is tender, 10 to 15 minutes.
Reduce heat; add cheese cubes and stir until melted. Stir in milk and garlic powder.
Stir cornstarch and water together in a small bowl until smooth. Stir into soup; cook, stirring frequently, until thick. Serve and enjoy!
