Caramelised shallot croque monsieur

Ingredients

• 30g unsalted butter, plus extra for buttering the toast
• 1 echalion shallot, finely diced
• 1 bay leaf
• 30g plain flour
• 280ml whole milk
• 1 tsp Dijon mustard, plus extra for spreading on the toast
• ¼ tsp ground nutmeg
• 4 slices of soft white bread
• 2 thick slices of good quality ham
• 150g grated Gruyere cheese