Wash watercress; reserve 8 small sprigs for garnish.
Trim off thick stems from remaining watercress; discard stems and set watercress aside.
In large saucepan, melt butter over low heat; cook onions 3 to 4 minutes, stirring often, until softened but not brown.
Stir in flour; cook, stirring, 2 minutes.
1 Gradually stir in chicken stock.
Increase heat to high; bring to boil, stirring constantly.
Add watercress and peas; return to boil.
Reduce heat to medium low; simmer, covered, 3 to 5 minutes or until peas are tender.
In food processor or blender, process soup in batches until smooth.
(Recipe can be prepared ahead up to this point.
Let cool, then refrigerate, covered, up to 2 days.)
Return soup to large saucepan; stir in half and half, salt, pepper and cayenne.
Heat through over low heat 5 minutes; do not boil.
If soup is too thick, add a little more half and half.
Taste and adjust seasoning.
Ladle into heated soup bowls; top each portion with a little sour cream and a reserved watercress sprig.