In a small bowl, whisk together the water, sugar, ketchup, hoisin, soy sauce, oyster sauce, sweet chili sauce, crushed red peppers and apple cider vinegar. Set aside.
After the beef has finished marinating, add the remaining cornstarch to the bowl. Toss the beef again to coat.
Heat the oil in a medium-sized skillet over medium-high heat. Once oil is hot, fry the beef in small batches for 2 to 3 minutes or until golden-brown. Place the fried beef slices on a large paper towel-lined plate. Cover to keep warm while you finish frying the rest of the beef in batches.
Heat a separate large skillet on medium-high heat and use two tablespoons of the oil you just fried the beef in. Add the onion and bell pepper to skillet. Cook for 2 to 3 minutes, until it just starts to caramelize on the edges. Add the garlic to skillet in and continue to cook a few more seconds until fragrant. Remove the veggies and place them with the beef on the plate.
Add the Beijing Sauce to the same skillet and cook on medium-high heat until it thickens, about 3 to 5 minutes. Add the beef and vegetables into the sauce and toss to combine. Serve immediately and enjoy!