In a medium bowl, combine the egg, 3 Tbsp cornstarch, salt and pepper. Mix well. Add the chicken and toss until each piece is fully coated. Set aside.
In a separate medium bowl, combine the soy sauce, brown sugar, vinegar, orange zest and juice, garlic and 1 Tbsp cornstarch. Whisk until cornstarch has dissolved.
Heat the canola or vegetable oil in a large skillet over medium-high heat. Add the chicken in a single layer (do this in multiple batches, if needed) and cook for 2 minutes per side, or until browned. Add all of the chicken back to the skillet, along with the orange sauce. Cook over medium heat until the chicken pieces are fully cooked through, 5-7 minutes.
Serve immediately over white rice. Top with green onions and orange zest.