Cook the onions and chopped cucumbers in 4 tablespoons of the
butter until the onions are transparent.
Add the broth or water and cook until the vegetables are tender.
Press through a sieve or whirl in a blender until smooth.
In the pan, melt the 2 remaining tablespoons butter and stir
in the flour until it is well blended. Pour in the pureed
vegetable mixture and stir vigorously until smooth. Cook until
thickened, stirring constantly.
Beat the yolks and cream together and stir into the hot soup.
Turn the heat down and simmer for 5 minutes. Just before
serving, stir in the diced raw cucumber, season with salt and
pepper, and garnish with the parsley.