In a large saucepan of 1 1/2 cups water, cook rice according to package instructions; set aside.
Place broccoli florets into a steamer or colander set over a pan of boiling water. Cover and steam for 5 mins, or until cooked through and vibrant green; set aside.
In a small bowl, whisk together chicken stock, orange marmalade, orange juice, soy sauce, vinegar, ginger, garlic, Sriracha, if using, and 1 tablespoon cornstarch.
Season chicken with 1/2 teaspoon salt and 1/2 teaspoon pepper; sprinkle with remaining 3 1/2 tablespoons cornstarch until evenly coated.
Heat oil in a large skillet over medium high heat. Working in two batches, add chicken and cook until golden brown, about 6-8 minutes. Stir in chicken stock mixture until thickened, about 2-3 minutes. Stir in sesame seeds.
Serve chicken immediately with rice and broccoli, garnished with green onion, if desired.