Heat 1/4 cup olive oil in a medium (10-11 inch) pot or Dutch oven, such as Le Creuset, over medium heat. Add the onions, carrots, celery, and fennel and saute for 10 to 15 minutes, stirring occasionally, until the vegetables are softened. Add the chicken stock, 2 cups water, the Parmesan rind, 1 tablespoon salt, and 1 teaspoon pepper. Bring to a boil, lower the heat, and simmer, partially covered, for 20 minutes, stirring occasionally. Remove the Parmesan rind and taste soup for seasonings.