In a mixing bowl, combine soy sauce, rice vinegar, sesame oil, and cornstarch to create a smooth marinade. Cut the boneless, skinless chicken breasts into bite-sized pieces, then toss them in the marinade, ensuring each piece is well-coated. Let the chicken marinate for at least 15 minutes while you prepare the other ingredients.

While the chicken is marinating, chop the scallions into thin slices, separating the green and white parts. Mince about three cloves of garlic and grate about a tablespoon of fresh ginger. Set these aromatics aside in separate bowls.

In a large skillet or wok, heat 2 tablespoons of vegetable oil over medium-high heat until it shimmers.

Add the marinated chicken to the hot skillet, spreading it out in a single layer. Sauté for 5-7 minutes, turning occasionally, until the chicken is golden brown and cooked through.

Once the chicken is cooked, stir in the minced garlic, grated ginger, and the white parts of the scallions. Sauté for about 2-3 minutes until the aromatics become fragrant.

Pour any remaining marinade into the skillet and let it simmer for another 2-3 minutes, stirring occasionally to thicken the sauce.

Remove the skillet from heat and let the chicken rest for a couple of minutes. Slice the chicken into strips, then serve over steamed rice or fresh noodles, garnished with the reserved green parts of the scallions.