My favorite place to eat Croque Madame or Monsieur (the difference being that the Monsieur is a closed sandwich while the Madame is open-faced) is in Café de Flore in St. Germain, in Paris. It’s a very simple, bistro-style café where the great artists and writers of Paris would come to share their thoughts.
Place the sliced bread under a broiler and toast one side. Remove from the broiler and butter the non-toasted side. Place half a slice of ham and a smear of Dijon mustard on each slice of bread. Top with equal amounts of grated Gruyère, then season with salt and pepper. Put back underneath the broiler until the cheese has melted.