Meanwhile, add 1 tsp sugar, vinegar, soy sauce, stock concentrate, remaining ginger and cornstarch, and ½ cup water to bowl with pineapple juice. Stir to combine. After emptying pan, heat a thin layer of oil in same pan over medium-high heat (we used 2 TBSP). Add chicken and cook, tossing occasionally, until almost cooked through, 5-8 minutes. TIP: Don’t overcrowd pan— work in batches if needed.