Start by cutting your boneless, skinless chicken breasts into bite-sized cubes.
In a large skillet, heat the vegetable oil over medium-high heat. Once hot, add the cubed chicken. Season with ground black pepper and sauté until the chicken is cooked through and golden brown, about 5-7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, lower the heat and add the minced garlic and freshly grated ginger. Sauté for about 30 seconds until fragrant. In a mixing bowl, combine the orange juice, low-sodium soy sauce, brown sugar, and water. Mix well.
In a separate small bowl, mix the cornstarch with a little water to create a slurry. Pour the orange juice mixture into the skillet and bring to a simmer. Stir in the cornstarch slurry and allow the sauce to thicken, stirring constantly.
Return the chicken to the skillet, tossing to coat in the thickened sauce. Let it simmer for another 2-3 minutes to heat through. Serve the Mandarin Chicken over a bed of steamed rice and steamed broccoli. Garnish with sliced green onions and sesame seeds.