Transfer the sliced flank steak to a large bowl and toss with the ½ cup of tapioca flour until all of the strips of beef are coated. Spray the bowl of your crock pot with cooking spray and move the beef to the bottom of your crock pot.
In a small bowl or a measuring cup, whisk together the coconut aminos, water (or broth), sesame oil, cider vinegar, minced garlic, minced fresh ginger, and black pepper.
Pour the sauce into the crock pot with the meat and give everything a big stir.
Secure the lid on the crock pot and cook on high heat for 2-3 hours or low heat for 4-5 hours, stirring halfway through if possible. Stir the meat and taste it for flavor. Season it to taste with sea salt.
Serve Mongolian beef over your choice of rice (white rice, brown rice, cauliflower rice, cilantro lime rice, etc.) with a side of steamed broccoli, chopped green onions and a sprinkle of sesame seeds, and enjoy!
