Peel the cucumber and slice it lengthwise. Scoop out seeds with a spoon 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 or ricer set over a large bowl. Force potatoes through. Return to saucepan. Stir in pepper, cream, milk, grated onion and the cucumber. Simmer gently for 5 minutes or until the cucumber is tender. Add dill and season to taste. Serve hot. Walt MM