In a medium bowl whisk together 1/3 cup orange juice, 1/4 cup low-sodium soy sauce, 2 tablespoons brown sugar, 2 tablespoons water, 1½ tablespoons cornstarch, 2 cloves minced garlic, 1/2 teaspoon freshly grated ginger, and 1/4 teaspoon ground black pepper until smooth and the cornstarch is fully dissolved.

Heat 1 tablespoon vegetable oil in a wok or large skillet over medium-high heat until shimmering.

Add 1½ pounds cubed boneless, skinless chicken breasts in a single layer (work in batches if needed to avoid overcrowding). Cook, turning or stirring occasionally, until the pieces are browned and mostly cooked through, about 2–3 minutes per side (4–6 minutes total depending on cube size).

Pour the prepared sauce over the chicken and stir to coat all pieces.

Bring the skillet contents to a low boil, then reduce the heat to low and simmer, stirring occasionally, for 5–8 minutes, until the sauce has thickened and the chicken is cooked through.

Serve the mandarin chicken hot with steamed rice and steamed broccoli; garnish with sliced green onions and sesame seeds.