Sauté onions in oil, 5 minutes, add the diced cucumbers. Blend corn starch with some cold chicken broth, add to cucumbers and add rest of broth. Add salt, pepper, basil and beau monde. Bring to boil, simmer for 25 minutes. Place in blender, blend and add lemon juice, sour cream. When blended add vermouth. Chill. Can be made several days ahead. When serving, garnish with chives.