In a greased 4- or 5-qt. slow cooker, combine the hash browns, broccoli, 3/4 cup cheese and onions.
In a small saucepan, melt butter. Stir in flour until smooth; gradually add milk. Bring to a boil; cook and stir for 1 minute or until thickened. Stir in the soup, salt and mustard. Pour over potato mixture; stir to combine.
Cover and cook on low for 4-5 hours or until potatoes are tender. Sprinkle with remaining cheese. Cover and cook 30 minutes longer or until cheese is melted.