Preheat oven to 375 degrees F.
Place potatoes in a saucepan; cover with water. Bring to a boil.
Reduce heat; simmer 20 minutes or until tender.
Drain potatoes in a colander over a bowl, reserving 1 cup cooking liquid.
Return potatoes and liquid to pan; mash with a potato masher until slightly chunky.
Add chopped broccoli and next six ingredients (broccoli through sour cream) to pan; stir well.
Spoon potato mixture into an 11 x 7 baking dish coated with cooking spray; bake at 375 degrees F for 35 minutes.
Sprinkle with cheddar cheese; bake an additional 5 minutes or until cheese melts.