Peel the cucumber and slice it lengthwise. Scoop out seeds with a sponn and discard. Dice cucumber. In a heavy 2 1/2-quart saucepan boil potatoes in salted water until the potatoes are very soft. Pour potatoes and cooking liquid into a sieve or food mill set over a large bowl. Force potatoes through. Return to the saucepan. Stir in pepper, cream, milk, grated onion and the cucumber. Simmer gently about 5 minutes or until the cucumber is tender. Add dill and season to taste. Serve hot.