Melt 1 tablespoon butter in large kettle, add leeks and onion and cook until tender but not browned. Add water and broth and bring to boil. Knead flour into remaining 3 tablespoons butter to form paste and stir into boiling mixture with wire whisk to blend well. Season to taste with salt and white pepper. Add potatoes, reduce heat and simmer until potatoes are soft. Press through sieve. Cool. Add cream and half and half to soup, then chill. Top each serving with sprinkling of chopped chives.