1/2 cup butter
2 yellow onions (peeled and roughly chopped)
8 leeks (white part only, washed and roughly chopped)
2 pounds fingerling potatoes (peeled and chopped)
  salt (to taste)
2 quarts vegetable or chicken stock
1 cup crème fraîche
  white pepper (to taste)
1 bunch fresh sorrel
1 teaspoon olive oil
1 bunch chives (snipped)