Melt butter in 3-quart saucepan over medium heat. Add onion and garlic; saute 5 minutes or until tender.
Stir in corn starch until smooth. Add broccoli, chicken broth, milk, salt and pepper. Bring to a boil, stirring occasionally, over medium heat and boil 1 minute. Remove from heat.
Carefully blend hot mixture 1/3 at a time in blender until smooth. (Blending too much hot mixture at one time can cause mixture to overflow from blender.)