While chicken cooks, combine ⅓ cup water, cornstarch, and soy sauce in a small bowl, whisking vigorously to get rid of any clumps. Reduce heat under pan used for chicken to medium-low and add a large drizzle of oil. Add garlic, minced ginger, and scallion whites. Cook, tossing, until fragrant and starting to soften, 2-3 minutes.