Croque Madame

Ingredients (9)

• 4 tbsp unsalted butter
• 2 tbsp all-purpose flour
• 1 cup whole milk
• 1 cup Gruyere cheese
• 5 tsp Dijon mustard
• Kosher salt and freshly ground black pepper
• 4 large eggs
• 8 slices good-quality white sandwich bread
• 1/2 lb thinly sliced Black Forest ham