In a medium bowl, whisk together soy sauce, rice vinegar, sesame oil, and cornstarch until smooth. Add the chicken breasts, ensuring they are well coated in the marinade. Cover with plastic wrap and let it marinate for at least 15 minutes.

Chop the scallions into thin slices, keeping the white and green parts separate. Mince a few cloves of fresh garlic and a small piece of ginger. Set these aside in a small bowl.

In a large skillet or wok, heat about 2 tablespoons of vegetable oil over medium-high heat until shimmering. Test the oil's readiness with a small piece of marinated chicken.

Add the marinated chicken to the skillet in a single layer. Cook for about 5-7 minutes on one side, then flip and cook for another 5-7 minutes until fully cooked.

In the same skillet, reduce the heat to medium and add the minced garlic, ginger, and the white parts of the scallions. Sauté for 2-3 minutes until fragrant.

If desired, pour the leftover marinade into the skillet, stirring to combine with the sautéed aromatics. Let it simmer for 2-3 minutes until slightly thickened.

Remove the skillet from the heat and let the chicken rest for a couple of minutes. Slice the chicken into bite-sized pieces and serve it over rice or noodles, drizzling the sauce on top.