In a small glass bowl, combine all the ingredients for the salsa and toss together gently. Cover and refrigerate for up to 4 hours before using. In a large heavy skillet, heat the butter over medium heat. Add the plantains with their flat sides down and saute for about 2 minutes, or until slightly softened but not brown. Turn to the other side, add the oregano, leeks and parsnips, cover the pan and cook until all the vegetables are tender, about 10 minutes, stirring occasionally. Transfer the vegetables to a food processor and puree until smooth, pulsing the machine and scraping down the sides of the machine as necessary. Add 2 cups of the chicken stock, puree to mix, and transfer the mixture to a medium saucepan. Whisk in the remaining chicken stock and the milk, add the lemon juice, and place the pan over mediumhigh heat. When the soup comes to a boil, reduce the heat to low, cover the pan and simmer for 10 minutes, stirring occasionally. Add the salt and pepper. Serve in shallow soup bowls, garnished with a dollop of the salsa.