Soak the peas overnight in a large bowl of cold water.
Heat the oil in a large pan, add the onion and sauté for 3-4 mins or until soft. Add the peas, stock and bay leaf and season with salt and pepper. Bring to the boil, then cover and simmer for 50-55mins or until the peas are tender. Discard the bay leaf.
Transfer the soup to a food processor or blender, add the gammon, 200ml water, the milk and pulse until the soup is nice and smooth. Adjust the seasoning to taste. Return to the pan and gently reheat until piping hot.
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