Melt the butter in a heavy saucepan; add the sliced leeks (or scallions), onion, and celery, cover pan, and cook gently for 10 minutes. Uncover and add carrots, potato, garlic, salt, pepper, sugar, and herbs. Mix well, add water to cover and simmer until the vegetables are perfectly tender (about 40 minutes). Liquidize in the blender (or puree), then return pureed mixture to the same saucepan. Add cornstarch and milk and simmer for 3 minutes, Taste for seasoning. Just before serving, stir in cream. Garnish with a good quantity of finely chopped parsley or mint and tiny croutons fried in butter (and garlic, if desired).