Place the cubed beef chuck roast into the slow cooker basin.
In a medium bowl, whisk together the beef broth, soy sauce, brown sugar, rice vinegar, sesame oil, ginger, garlic, and red pepper flakes.
Pour the liquid mixture over the beef in the slow cooker and stir to coat.
Cover and cook on low heat for 7 to 8 hours or high heat for 4 hours until the beef is fork-tender.
Using two forks, shred the beef within the slow cooker.
Add the dry ramen noodles and baby spinach to the slow cooker, submerging them in the liquid as much as possible.
Cover and cook on high for an additional 5 to 10 minutes until the noodles are tender.
Toss to combine all ingredients and garnish with sliced green onions before serving.