In a large bowl (or ziplock bag) add beef slices and cornstarch and mix to thoroughly coat beef. Set aside.
Wash orange thoroughly. Use a microplane or grater to zest about 2 tsp orange zest. (Alternatively use a vegetable peeler to cut a large strip of orange peel and use a pairing knife to cut strip into very thin slices). Set zest aside.
Cut orange in half and squeeze juice into a cup (you’ll need ¼ cup orange juice total). In a medium bowl mix ¼ cup orange juice, soy sauce, rice vinegar, brown sugar, ginger, garlic and red pepper flakes. Whisk to combine.
Add sesame oil to a large skillet over medium-high heat. Once oil is hot, add beef strips making sure to leave space around each strip (so beef browns and doesn’t “steam”). Allow beef to cook for approx. 2 minutes untouched. Flip beef over (with tongs) and allow to cook an additional 2 minutes. It may be necessary to work in batches if skillet is not large enough to cook all of the beef strips at one time.
Reduce heat to medium-low, add back beef if you removed any to cook in batches, and add sauce. Allow to cook approx. 5 minutes, until sauce reduces and becomes a sticky glaze.
Garnish with orange zest, sliced green onions and toasted sesame seeds.
Serve over rice, cauliflower rice, or in lettuce cups.