Salmon, leek and asparagus lasagne

Ingredients

• 600 grams pin-boned skinless salmon fillets
• 125 grams chopped butter
• 3 large, pale section only, washed, thinly sliced leeks
• 1/2 cup plain flour
• 4 cups milk
• 1 cup finely grated parmesan
• 1/4 cup parsley, chopped
• 2 teaspoons dijon mustard
• 2 teaspoons finely grated lemon zest
• 200 grams fresh lasagrie sheets
• 3 bunches, trimmed, halved lengthways asparagus
• baby spinach,150g
• 1/2 cup grated tasty cheese
• mixed salad leaves, to serve