Peel the cucumbers, cut them lengthwise into halves, and remove the seeds. Place the cucumber halves in boiling salted water to cover and simmer for three minutes. Drain and then chop.
Melt the butter and blend in the flour; stir in two cups stock. Bring the mixture to a boil and add the chopped cucumbers, salt and pepper. Continue to simmer for about twenty minutes, until the cucumbers are tender.
Force the mixture through a fine sieve or purée in an electric blender. Correct the consistency by adding more stock, if needed, and adjust the seasoning. Reheat the soup and serve garnished with croutons.