In a pot over medium heat, melt the butter. Add the flour and cook for 1 minute while stirring. Whisk in the milk and bring to a boil. Simmer over medium-low heat for 5 minutes or until the sauce has thickened, stirring regularly. Season with salt and pepper. Cover and set aside.
                        
                      
                    
                                                                                    
            
                                          
                            Croque-Monsieur 
                            
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