Cook the noodles until al dente as directed on package. Drain well. Combine the noodles, broccoli, onion, and cheese in the crock pot. In a small bowl, combine the flour and milk until smooth. Stir into the crock pot. Cover the crock pot and cook on low heat for 4 hours, stirring once or twice during the cooking time. Add additional milk as needed until the soup is the desired consistency. Stir well before serving and adjust the seasoning as needed with salt and pepper.