1. Cook the potatoes in boiling water for 15 minutes until just cooked, adding the asparagus to the pan 2 minutes before the end of cooking. Drain and, once cool enough to handle, peel the potatoes and halve the asparagus; set aside.
2. Put the clams in a pan with the white wine. Cover, shake gently and cook for 3–4 minutes over a high heat until the shells have opened. Do not shake too vigorously as the clams can easily fall from their shells. Discard any clams that have not opened.
3. In a large pot, melt the butter and add the onion, celery and leek. Cover and cook for 12–15 minutes over a gentle heat until soft and translucent. Add the stock and any clam juices, and bring to a gentle simmer.
4. Put the fish fillets in the pot and shake lightly to let them settle. Pour over the lemon juice, cover and gently simmer for 5 minutes, then spoon over the clams and any remaining juice. Tumble in the potatoes and asparagus, and scatter over the parsley. Return the lid to the pot, count to 30 and take to the table.