In a medium bowl, combine soy sauce, water, brown sugar, oyster sauce, sesame oil, minced garlic, and grated ginger. Stir until the sugar is dissolved to make the sauce.
Place the thinly sliced beef flank steak into a gallon-sized freezer bag.
Pour half of the prepared sauce into the freezer bag with the beef. Seal the bag, remove excess air, and mix to coat the beef evenly. Freeze the bag if not cooking immediately.
When ready to cook, thaw the freezer bag overnight in the refrigerator.
Transfer the beef and marinade into the crock pot.
Cook on low for 4 hours.
About 30 minutes before serving, add the fresh broccoli florets to the crock pot and stir gently to combine.
In a small bowl, mix cornstarch and cold water until smooth to create a slurry.
Pour the remaining half of the sauce and the cornstarch slurry into the crock pot. Stir gently to combine.
Cook on low for an additional 30 minutes until the sauce thickens and broccoli is tender-crisp.
Serve hot over steamed rice or noodles as desired.