Prepare the Orange Sauce first. Combine all of the sauce ingredients in a bowl and whisk to combine. Set aside.
Place sliced beef in a large plastic bag. Add the soy sauce, close the bag, and toss to coat the beef. Then add the cornstarch and a little salt and pepper. Close the bag again and toss to coat the beef.
In a wok (or large pan good for frying) add enough oil so that there is an inch or two of oil. Heat the oil over high heat. Add the beef in batches, carefully flipping occasionally while frying. Fry for a few minutes until crispy and then remove to a plate lined with paper towels. Continue frying in batches until all of the beef has been fried.
Carefully discard all of the oil except about 2 tbsp. Over medium-high heat, add the red pepper and onion to the wok. Cook the veggies until tender-crisp, about 3-4 minutes.
Add the orange sauce to the wok. Stir frequently until the sauce has thickened, about 2-3 minutes. Then add the beef back to the skillet and toss in the sauce. Remove from heat, add the green onions (reserve some for serving), and toss.
Serve over rice (optional) and top with a little extra orange zest and sliced green onions. Enjoy!