Wash, pare, and cut potatoes in halves. Wash, pare, and cut turnips in one-quarter inch slices. Parboil together ten minutes, drain, add onion cut in slices, and three cups boiling water. Cook until vegetables are soft; drain, reserving the water to add to vegetables after rubbing them through a sieve. Add milk, reheat, and bind with butter and flour cooked together. Season with salt and pepper.