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 6 ingredients (broccoli through sour cream) to pan, and stir well. Spoon potato mixture into an 11- x 7-inch baking dish coated with cooking spray; bake at 375° for 35 minutes. Sprinkle with cheddar cheese; bake an additional 5 minutes or until cheese melts.