Over medium heat in a small saucepan, melt butter and whisk in flour. Let cook 2-3 minutes. Whisk in milk until smooth and season. Stir frequently and let thicken. Spray the basin of a 6 quart slow cooker. Pour in the broccoli pieces and top with the gravy. Stir to combine. Cover & cook on HIGH for 1 hour or LOW for 2 hours. Stir. Replace cover and cook an additional 30 minutes or until broccoli is tender. Each slow cooker cooks differently so your cook time may vary. Once broccoli is tender, sprinkle in the cheese, stir, and then top with the fried onions. Cook, uncovered, for an additional 10 minutes or until cheese is melted.