Broccoli Cheese Soup

Ingredients: 
 1/2 C butter 
1 onion, chopped 
1 lb chopped fresh broccoli (you can use frozen as well) 
3 cans chicken broth (43.5 oz) 
1 lb processed cheese (such as Velveeta) cubed 
2 cups milk 
1 tablespoon garlic powder
 2/3 C cornstarch 
1 C water 
1/2 C cheddar cheese, plus more for serving 

  Instructions: 
-In a stockpot, melt butter over medium heat. Cook onion in butter until softened. Stir in broccoli, and cover with chicken broth. Simmer until broccoli is tender, 10 to 15 minutes. 
 -Reduce heat, and stir in cheese cubes until melted. Add in cheddar cheese. Mix in milk and garlic powder. 
-In a small bowl, stir cornstarch into water until dissolved. Stir into soup; cook, stirring frequently, until thick.