In the bottom of a slow cooker, combine the miso paste, ginger, garlic, tamari, rice vinegar, honey, mirin, and orange juice. Whisk until well combined and smooth.
Sprinkle both sides of the chuck roast lightly with salt and pepper. Add to the slow cooker and flip the roast a few times so it is well coated in the sauce.
Cover and cook on low for 8 to 10 hours, or on high for 4 to 6.
When the cook time is complete, transfer the chuck roast to a rimmed sheet pan and use two forks to shred the meat into large pieces. Transfer the meat and its juices back to the slow cooker and toss so that it is well coated in the sauce.
To serve, divide the beef over the prepared rice and drizzle with additional sauce, if desired. Top with green onions, if desired.
Notes
Orange Juice Note: You can either juice half of a medium orange or juice one clementine! Both work great here.