Pre-heat oven to 200°C (400°F, gas mark 6). Slice the leeks into thick circles (no larger than 2cm) and place in a large saucepan with the stock, cream and mustard. Cook gently for 15 mins until starting to soften.
Transfer to a ceramic baking dish, saving 2 or 3 tbsp of the stock mixture.
Place the breadcrumbs, nuts and parsley in a food processor and whizz together until finely chopped.
Scatter the crumble and cheese over the leeks and drizzle the remaining stock mixture over the top. Bake for 10-15 mins until starting to turn crisp and golden. If necessary, finish under the grill.