Prep everything before cooking: cut chicken into bite-size pieces, mince garlic, grate ginger, and zest the orange.
Place chicken in a medium bowl, season with salt and pepper, then sprinkle 3 tablespoons cornstarch over the chicken and toss until evenly coated.
In a separate bowl, whisk together orange juice, soy sauce, lemon juice, packed brown sugar, 1 tablespoon cornstarch, orange zest, grated ginger, minced garlic, and red pepper flakes (if using).
Heat 2 tablespoons olive oil in a skillet over medium-high heat until hot.
Add half of the coated chicken to the skillet in a single layer without crowding; cook about 4 minutes, flip, then cook another 3–4 minutes until browned and cooked through to 165°F. Transfer to a plate.
Add remaining 1 tablespoon olive oil to the pan and cook the second batch of chicken the same way; transfer to the plate when done.
Turn off the burner, pour the sauce into the hot skillet, and return the heat to medium-high; as the sauce bubbles, stir and scrape up browned bits until it thickens, about 1 minute.
Add the cooked chicken back to the skillet and toss to coat thoroughly in the sauce. Serve immediately, garnished with chopped scallions if desired.