Croque Monsieur (sandwich ham dan keju panas Prancis)

Bahan-bahan (2 porsi)

• 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)
• 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))
• 120 g / 4 oz ham slices (, preferably smoked)
• 4 tsp Dijon mustard
• 30 g / 2 tbsp unsalted butter
• 1/2 cup gruyere cheese (, shredded (packed cup, Note 2))
• 3 tbsp parmesan (, finely shredded (Note 2))