Check with your fishmonger whether the clams have been cleaned. If you need to clean them yourself, put them into a bowl of cold water with a tablespoon of fine salt and leave them for 40 minutes, moving them around gently every 10 minutes. The salt water makes them expel any sand.
Pod and wash the fresh coco beans.
Put the olive oil into a large saucepan on a medium-low heat. When hot, add the garlic and onions and cook for 10–12 minutes. Add the leek and cook for 5–6 minutes, then add the peppers, bay leaf and dried chillies. Cook for around 8–10 minutes, until really soft and sweet but not coloured, then add the wine. Cook until the wine has evaporated, then add the stock and the coco beans. Half cover with the lid and steam very gently until the beans are nice and soft.
When the beans are ready, season with salt and pepper and add the clams. Half cover with the lid again, and as soon as the clams start to open, add the chopped chervil and mix everything together, seasoning again.
Serve with lots of bread, for mopping up the sauce.