Prepare the flank steak by slicing it thinly against the grain, then cutting it into dices. Place the diced steak in a bowl.
Season the steak with salt and pepper, then sprinkle with cornstarch. Toss until the steak pieces are evenly coated.
Mince the garlic and ginger.
In the slow cooker, layer the vegetable oil and the cornstarch-coated steak.
Add the minced garlic, ginger, brown sweetener, sesame oil, coconut aminos, and water to the slow cooker. Stir to ensure all ingredients are well mixed.
Set the slow cooker to cook for 2.5 hours on high or 4-5 hours on low. Stir the beef once it's finished cooking to blend the flavors.
While the beef cooks, slice the scallion into thin pieces.
Once the Mongolian beef is ready, garnish it with the sliced scallions and sprinkle sesame seeds over the top.
Serve the Mongolian beef over a bed of rice for a complete meal.