Place first 4 ingredients in blender and blend at high speed until onion is pureed.
Add next 4 ingredients and blend again.
Add cream, 1/3 at a time, and blend until very smooth.
Pour into 1-quart mold and chill overnight.
To unmold, dip bottom and sides of mold into warm water and invert onto a serving plate.
Decorate mousse with parsley.