Place the cornstarch and black pepper in a large resealable plastic bag, add the chicken pieces a few at a time, and shake to thoroughly coat.
Heat 1 tablespoon oil in a medium saucepan over medium heat. Add the minced garlic and the white parts of the scallions and sauté until fragrant, about 30–60 seconds, taking care not to burn.
Add the tomato sauce, water, honey, chili garlic sauce, soy sauce, and orange juice to the pan. Bring to a boil, then reduce heat and simmer for 5–6 minutes until slightly reduced.
In a wok or large sauté pan, heat the sesame oil and 1/3 cup vegetable or canola oil over medium-high heat. Working in batches to avoid crowding, brown the coated chicken pieces on all sides, about 4–5 minutes per batch. Transfer cooked chicken to a paper-towel-lined plate.
Wipe out the wok or sauté pan, return the browned chicken to the pan, and pour the prepared sauce over the chicken. Cook and stir until the sauce thickens slightly and coats the chicken, about 2–3 minutes.
Serve the sesame orange chicken over hot cooked rice and garnish with sliced scallion tops and toasted sesame seeds if desired.
