Slice beef into thin strips.
Add beef to the slow cooker.
Pour in beef broth and soy sauce.
Add hoisin, brown sugar, and sesame oil.
Mix in vinegar, ginger, and garlic.
Sprinkle red pepper flakes if using.
Stir everything gently to combine.
Cover and cook on low 6 hours.
After 6 hours, add mushrooms and carrots.
Cook for another 30 minutes.
Boil ramen noodles separately, drain well.
Stir noodles into slow cooker.
Add baby spinach and green onions.
Let sit for 5 minutes before serving.
Garnish with sesame seeds and enjoy.