Foryoueats: Croque Monsieur (French hot ham and cheese sandwich)

Ingredients

• Bechamel:
• 1/2 cup milk
• 1/2 cup cream (pure or heavy / thickened)
• 1 1/2 tbsp / 25g unsalted butter
• 1 1/2 tbsp flour , plain / all-purpose
• 1/4 tsp salt , kosher/cooking salt
• 1 pinch white pepper
• 1/8 tsp nutmeg , preferably freshly ground
• Sandwich:
• 4 slices sourdough bread , 1.5cm / ⅗" thick (Note 1)
• 8 slices Swiss or gruyere cheese (165g / 6 oz, enough for 2 layers a sandwich) (Note 2)
• 120g / 4 oz ham slices , preferably smoked
• 4 tsp Dijon mustard
• 30g / 2 tbsp unsalted butter
• Topping:
• 1/2 cup gruyere cheese , shredded (packed cup, Note 2)
• 3 tbsp parmesan , finely shredded (Note 2)