In a saucepan, bring chicken broth to a boil. Add broccoli, onion and bay leaves. Reduce heat and simmer until broccoli is tender; remove bay leaves. Meanwhile, in another saucepan, melt butter. Stir in flour to make a smooth paste. Gradually stir in milk. Cook over medium heat until mixture is hot and thickened, stirring occasionally. Add 1 cup of broccoli stock to milk mixture; stir until well blended. Gradually add remaining broccoli stock to milk mixture. Heat and stir until well-blended. Season with salt and pepper.