In a small bowl, whisk together soy sauce, water, brown sugar, red pepper, vinegar, garlic, and ginger.
Slice the flank steak against the grain into 1/2" strips and place in slow cooker. Add onions, toss to combine. Pour sauce over top.
Cover and cook on LOW 6 hours or HIGH for 3-4.
At the end of cooking, you have two options- remove a small amount of cooking liquid and whisk it into the cornstarch in a separate small bowl until smooth, then return it to the slow cooker, stir in and cook on HIGH for 45 minutes to thicken the sauce; or, pour off all cooking liquid into a saucepan, whisk in the cornstarch and simmer on the stove until thickened.
Gently stir together the beef and onions, sauce and green onions. Serve over white rice.