Cut the leeks into rings and wash thoroughly. Dice the potatoes. Sweat the leeks in 3 tablespoons of olive oil, add the potato and stir into the leeks for a few minutes. Add the stock and milk. Season and gently simmer for 45 minutes, or until potatoes are cooked. Transfer the soup to a liquidizer, add the cream and liquidize for 1 minute. Return the soup to the heat, bring to just under boiling point. Divide the soup between 6 deep soup bowls, put toasted bread on top and cover with cheese. Pass lightly under a hot grill. Shave white truffle on top.