Heat oil in a skillet over high heat; brown chicken in two batches, 6 to 8 minutes. Set aside.
In same skillet over medium heat, cook ginger, onion, and garlic, stirring until browned, 8 to 10 minutes. Add soy sauce, vinegar, and sugar; cook over high heat until thick, 3 to 4 minutes. Add chicken; stir to warm. Remove from heat; stir in scallions.