Pat the chicken dry. In a bowl, toss chicken with cornstarch, garlic powder, salt, pepper, and 1 tablespoon soy sauce.
Let it sit for 10 minutes while you make the sauce.
In a small bowl or measuring cup, whisk together orange juice, zest, soy sauce, honey, vinegar, sesame oil, garlic, ginger, and red pepper flakes if using. Taste and adjust sweetness or salt as needed.
Heat a large nonstick skillet over medium-high. Add oil.
Spread chicken in a single layer. Cook 6–8 minutes, turning occasionally, until golden on the edges and cooked through. Work in batches if your pan is crowded.
Lower heat to medium.
Pour in the orange sauce. Simmer 2–3 minutes until it begins to reduce.
Stir the cornstarch slurry again and drizzle it into the skillet while stirring. Simmer 1–2 minutes until the sauce turns glossy and coats the chicken.
If too thick, add a splash of water. If too thin, simmer another minute.
Turn off heat. Sprinkle with sesame seeds and green onions if you like.
Portion cooked rice and veggies into 4 meal prep containers.
Top with the orange chicken and any extra sauce from the pan.