In a medium pot, bring 3 cups water to a boil, add the kale and boil for 10 minutes. Remove kale from broth, put it in a blender and set aside. Take broth off heat and set aside.
In a separate small skillet, heat olive oil and saute chopped onions over medium heat for 5-7 minutes. Add onions, ginger, prosciutto, cayenne and salt and pepper to the kale in the blender, and puree. Pour the pureed ingredients into a large pot, add milk and bring to a boil. You may wish to add 1/4 cup or so of the reserved broth to reach your desired consistency for soup.
Set out 4 soup bowls and pour 1 tablespoon of sherry into each bowl. Ladle soup into bowls and serve.