In a slow cooker, combine the beef chuck roast, beef broth, water, soy sauce, hoisin sauce, sesame oil, garlic powder, ginger powder, and black pepper. Stir to mix well.
Add the sliced mushrooms and half of the chopped green onions to the slow cooker. Cover and cook on low for 6-8 hours or until the beef is tender.
About 30 minutes before serving, add the instant ramen noodles to the slow cooker, pushing them down into the broth. Cover and let cook until the noodles are tender, about 5-7 minutes.
Stir in the baby spinach and let it wilt for a few minutes.
Serve the beef ramen in bowls, garnished with the remaining green onions and sesame seeds.