Preheat the oven to 375ºF.
In a bowl, place the beets, fennel, 2 tablespoons of the oil, salt, and pepper, and toss to combine. Spread the vegetables on a baking sheet and roast until caramelized, about 1 hour.
Remove from the heat and let cool slightly. When cool enough to handle, peel beets and cut into 1/2-inch cubes.
In a medium, heavy pot, heat the remaining oil over medium-high heat. Add the onions and sauté until soft, about 4 minutes. Add the carrots and celery, and sauté until soft, about 3 minutes. Add the garlic and sauté for 30 seconds. Add the fennel, beets and stock, and bring to a boil. Reduce the heat to medium-low and simmer for 20 minutes. Remove from the heat and let cool slightly.
With a hand-held immersion blender, or in batches in a food processor or blender, puree the soup. Refrigerate the puree until cool, at least 2 hours.
In a small bowl combine the sour cream, apple sauce, mint, and sage and stir to combine.
Ladle the chilled soup into serving bowls, top with a dollop of Apple-Sage-Mint Crema and some of the crushed pistachios, and serve.
Yield: about 6 cups (4 to 6 servings)