In a mixing bowl, combine sliced boneless, skinless chicken breasts with soy sauce, mirin, and cornstarch. Toss the chicken to ensure it’s evenly coated with the marinade. Allow it to sit for about 15 minutes.

In a food processor, combine the green parts of scallions, garlic cloves, and peeled fresh ginger slices. Pulse until finely chopped and fragrant, about 30 seconds.

Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Once the oil shimmers, add the marinated chicken in a single layer, cooking for about 3 minutes until golden brown. Flip the chicken pieces and continue cooking for another 2 minutes until fully cooked through.

In the same skillet, add another tablespoon of vegetable oil and heat it. Add the chopped scallion mixture and sauté for 30 seconds until fragrant. Add the baby bok choy, mushrooms, and snow peas, cooking for about 2 minutes until crisp-tender.

Return the chicken to the skillet with the vegetables. Pour in additional soy sauce, mirin, and 1/4 cup water to create a savory sauce. Toss everything together for about 2 minutes.

Taste the stir-fry and season with salt if necessary. Serve the hot mixture over freshly cooked rice or noodles.