Smoked Trout Lasagne

Ingredients

• 100g of unsalted butter, plus extra for greasing
• 100g of plain flour
• 950ml of whole milk
• 1 tbsp of Dijon mustard
• 1/2 tsp grated nutmeg
• 1/2 leek, cut into thin strips
• 400g of baby spinach
• 2 garlic cloves, crushed
• 10 fresh lasagne sheets
• 500g of cold smoked trout, cut into 2cm squares
• 250g of cheddar, grated
• 50g of Parmesan, grated
• 4 sprigs of dill, chopped
• Salt
• Pepper
• Salad leaves, to serve