Combine the farro and stock in a large, heavy saucepan over medium heat.  Cover and simmer, stirring occasionally, until the farro is tender (about 45 minutes).  Meanwhile, heat the olive oil in a skillet over medium high head.  Add the onions and sauté for 5 minutes, until the onions start to soften.  Stir in a few pinches of salt.  Once soft, puree them in a food processor (just a pulse or two ~ you want to maintain a few chunks).  When the farro is nearly cooked, stir in the asparagus.  Let the pot simmer for another few minutes, until the asparagus is a vibrant green.  Some stock will still be visible in the pot, and that is fine.  Stir in the lemon zest, walnuts and the 1/2 teaspoon of salt.  Add more salt to taste if needed.  Ladle into bowls and garnish with a dollop of creme fraiche (if using), a dusting of parmesan, and a hearty spoonful of the onions.