Add the broth, tamari, ginger, garlic, rice vinegar, Sriracha, and mirin to the slow cooker and stir. Add the roast and mushrooms. Set the slow cooker to low and cook for 6-8 hours or until the roast is cooked through and falling apart.

Remove the roast from the slow cooker and shred the beef off the bone. Return the meat to the crock pot. Add the bok choy to the soup about 10 minutes before serving, allowing it to wilt slightly.

Meanwhile, cook the udon noodles according to package directions. Drain.

Ladle the soup into bowls and add the noodles (as much or as little as you prefer) to each bowl. Top with radishes if desired and serve immediately.