Into a small bowl, grate the zest from 1 orange and squeeze the juice from 2 oranges. Add the garlic and soy sauce to the bowl. Mix the cornstarch with one tablespoon of cold water and add the cornstarch mixture to the bowl.
Heat 1 tablespoon oil in a large nonstick skillet over high heat. Working in batches (adding more oil if needed), brown beef on all sides, 3 to 5 minutes; season the meat with salt and pepper and transfer to a plate.
Pour juice mixture into skillet, and boil until syrupy, about 1 minute. Return beef to skillet; add orange segments and green onions. Toss until coated and heated through. Top with sesame seeds and serve with steamed rice if desired.