For mushroom stock, combine porcini mushrooms and beef stock in a large saucepot. Bring to a simmer, remove from heat and let stand 10 minutes. Strain mushrooms from pot and set aside.
Meanwhile, heat 2 tbsp of olive oil in a large skillet over medium-high heat. Season oxtails with salt and pepper. Cook 18-20 minutes or until browned on all sides. Set oxtails aside. Drain oil; add wine and scrape bits from bottom of pan. Add wine and oxtails to mushroom stock. Simmer over low heat for 3½-4 hours or until oxtails are completely tender. Remove oxtails from saucepot and chill until cool enough to handle.
Meanwhile, place herbs, bay leaves and peppercorns into a small square of cheesecloth. Fold cheese cloth around herbs to make a sachet and tie with butcher’s twine. Shred meat from oxtails and discard remaining bones and cartilage.
In a medium saucepot, combine remaining oil, onions, parsnips, carrots, cremini mushrooms and celery. Cook over medium heat 16-18 minutes or until vegetables are soft. Stir in tomato paste. Add mushroom stock, chicken stock, water, 5-Grain Blend, oxtail, sachet and beans. Simmer 20 minutes.
Ladle soup into serving bowls. Drizzle with truffle oil, if desired.