Cut the chicken into bite-size pieces and season with salt and pepper in a medium bowl.
Sprinkle 3 tablespoons cornstarch over the chicken and toss until each piece is evenly coated.
Whisk together the sauce: 3/4 cup orange juice, 2 tablespoons soy sauce, 1 tablespoon lemon juice, 1/4 cup packed brown sugar, 1 tablespoon cornstarch, zest from 1 orange, 1/2 teaspoon grated ginger, 2 minced garlic cloves, and 1/4 teaspoon crushed red pepper flakes (if using).
Heat 2 tablespoons olive oil in a skillet over medium-high heat until hot.
Working in a single layer, add half the chicken to the hot skillet without crowding and cook until browned, about 4 minutes, then flip and cook another 3–4 minutes until cooked through (165°F).
Transfer the cooked chicken to a plate, add about 1 tablespoon more oil to the pan, and cook the remaining chicken the same way.
Turn the burner off, pour the sauce into the hot skillet, and return the heat to medium-high; the sauce will thicken within about a minute as it bubbles—stir and scrape up any browned bits.
Add the cooked chicken back to the skillet and toss to coat completely in the sauce.
Serve immediately, topped with chopped scallions if desired.
