Croque Madame

Ingredients

• 5 tablespoons unsalted butter
• 3 tablespoons plain flour
• 2 cups whole milk
• 1/4 teaspoon salt
• 1/8 teaspoon black pepper
• 1/8 teaspoon freshly grated nutmeg
• 100g coarsely grated Gruyère cheese
• 8 slices firm white sandwich bread
• 4 teaspoons Dijon mustard
• 225g thinly sliced leg ham
• 4 large eggs