Slice the leeks into circles (no thicker than 2 cm) and place in a large saucepan with the stock, cream and mustard. Cook gently for 15 minutes 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 to 15 minutes until starting to turn crisp and golden. If necessary, finish under the grill.
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