Heat oil in heavy-bottomed soup pot. Add onions and potatoes; cook over medium heat until they start to soften and slightly brown. Toss in the watercress, ginger, salt, pepper, and one-inch piece of lemongrass. Heat until watercress wilts, approximately two to three minutes. Deglaze with stock, scraping up flavor bits from bottom of the pot. Simmer until potatoes are fork tender (approximately ten more minutes).
Remove from heat, dispose of lemongrass piece, and puree mixture until smooth. Return to heat and add cream. Whisk until smooth. Pass bisque through strainer to create silky texture.
Fill each eggshell with equal portions of bisque. Garnish with creme fraiche, watercress leaves, and remaining lemongrass (that has been finely chopped and sauteed until crisp).
*Using a paring knife, score the eggshells two-thirds of the way up the side. Carefully insert the sharp knife tip into the scored line (creating a small hole); repeat three more times around the shell, then, again very carefully, insert the tip of the knife into one of the holes and pull up, cracking and removing the top. Remove egg contents (save for another use). Add eggshells to boiling water and cook for three to four minutes. Remove shells and drain, making sure to wipe out any remaining membrane inside. Fully dry before using.