Begin by preparing all components before cooking, as the process is fast-paced. Dice the chicken into small, uniform chunks and place them in a medium bowl. Season lightly with salt and pepper, then dust with cornstarch. Mix to ensure even coating on all sides.
In a separate bowl, combine the orange juice, brown sugar, soy sauce, lemon juice, orange zest, cornstarch, grated ginger, minced garlic, and red pepper flakes (if using). Whisk until smooth and set aside.
Warm 2 tablespoons of olive oil in a large skillet over medium-high heat. Once the oil is hot and shimmering, add half of the chicken, spacing the pieces apart for proper browning. Cook undisturbed for 4 minutes, flip, and continue cooking for another 3 to 4 minutes, until golden and the internal temperature reaches 165°F. Remove the chicken and repeat the process with the remaining oil and chicken, transferring all cooked chicken to a plate when finished.
Switch off the heat and carefully pour the prepared sauce mixture into the hot skillet. The residual heat will cause the sauce to bubble and thicken in about a minute. Stir constantly, loosening any caramelized bits from the pan base.
Return the chicken to the skillet and toss thoroughly to coat each piece in the glossy sauce. Serve immediately while hot, optionally topped with sliced scallions.