To make the teriyaki sauce, in a saucepan, combine ½ cup soy sauce, mirin, brown sugar, 2 minced garlic cloves, and 1 teaspoon grated ginger.
Simmer for 5 minutes until slightly thickened. Let cool slightly.
Marinate the chicken in half the teriyaki sauce for 1 hour.
Whisk together the peanut butter, vinegar, sesame oil, 1 tablespoon soy sauce, 1 minced garlic clove, 1 teaspoon grated ginger, and enough warm water to reach a pourable consistency.
Preheat a pan with oil to medium heat.
Sear the chicken for 5-6 minutes per side until cooked through.
Let cool fully, about 20 minutes.
Slice the chicken into strips.
Arrange the broccoli, carrots, and cabbage on a plate.
Top with the chicken slices.
Sprinkle with the remaining teriyaki sauce, peanut dressing, peanuts, and scallions.
Toss together and serve the salad chilled or at room temperature.