In a medium bowl, whisk together soy sauce, beef broth, brown sugar, sesame oil, minced garlic, and grated ginger until well combined.
Place the thinly sliced beef flank steak into a large resealable freezer bag.
Pour the soy sauce mixture over the beef in the freezer bag, seal, and gently massage to coat the beef evenly.
Lay the bag flat and freeze for up to 3 months.
When ready to cook, thaw the freezer bag overnight in the refrigerator.
Spray the crock pot insert with vegetable oil or lightly grease it.
Transfer the beef and marinade from the bag into the crock pot.
Cook on low for 4 hours.
About 30 minutes before serving, add the broccoli florets to the crock pot and stir gently.
In a small bowl, mix cornstarch and water to create a slurry.
Pour the slurry into the crock pot and stir to thicken the sauce.
Continue cooking on low for the remaining 30 minutes until the broccoli is tender and the sauce has thickened.
Serve hot over steamed rice or noodles if desired.